Definify.com

Webster 1913 Edition

Noun.

One who, or that which, clinches; that which holds fast.

Pope.

That which ends a dispute or controversy; a decisive argument.

Noun.

That which clinches; a cramp or piece of iron bent down to fasten any thing.

One who makes a smart reply.

That which makes fast.

clincher ‎(plural clinchers)

That which clinches; that which makes something final or firm.
The clincher was that we couldn't wait any longer to leave, or it would get dark.
- 2011 December 10, Arindam Rej, “Norwich 4 - 2 Newcastle”, in BBC Sport:
  Newcastle's Dan Gosling another sent off for a foul on Russell Martin but Ba fired in to cut the deficit, before Holt nodded in the clincher.
(cycling) A tyre with a bead around the edge to attach to the rim of the wheel when inflated.