Josh Staunton was sent-off at his former club after 35 minutes: before that Town had edged the game and kept Dagenham quiet, after it, The Shaymen were forced to try and preserve a point against the odds.
That task was made even harder when Jack Munns scored on the hour mark, but Town’s fighting spirit was rewarded when Clarke rose brilliantly to head in a corner with 10 minutes to go.
Dagenham also finished the game with 10 men when captain Ben Nunn got a second booking.
A poor first-half was only brought to life when Staunton was dismissed 10 minutes before the interval.
The former Dagenham man certainly lunged for the ball, and caught Alex McQueen near the halfway line after winning it.
The challenge looked reckless, but whether it was of sufficient force and malice to warrant a red rather than a yellow card was unclear on first sight.
Town could hold out no longer than the hour mark, when Munns found a yard of space on the left of a crowded penalty box, not helped by the again impressive Jacob Hanson’s unfortunate slip, and drilled a shot back across goal into the top right corner.
But the hosts were made to pay for not making more of their upper-hand when Clarke met Sellers’ corner with a terrific header at the far post.
Dagenham captain Nunn was then sent-off for a second booking in added time.
Elsewhere, Full-back Warren Burrell scored a hat-trick as Harrogate came from behind to beat Aldershot 4-1 and close the gap on the the National League leaders.
Adam McDonnell opened the scoring for the visitors in the 14th minute but Harrogate levelled shortly after the half-hour mark when Burrell squeezed the ball in from close range after a goalmouth scramble.
Burrell added his second in the 54th minute to put Town ahead for the first time in the match when he headed in Joe Leesley’s corner.
He then notched his third in the 70th minute when he headed in Leesley’s free-kick.
Jack Muldoon added the gloss to the scoreline when he fired into the top corner in stoppage time.
The win kept third-placed Harrogate in touch with leaders Leyton Orient and second-placed Salford, who both lost on Saturday.
In National League North, York City lost 2-0 at Boston United, while Bradford Park Avenue drew 2-2 with Southport.
Guiseley’s game with Hereford was postponed due to a waterlogged pitch.