Mount tailless deer (4)
I believe the answer is:
stag
'deer' is the definition.
(stag is a kind of deer)
'mount tailless' is the wordplay.
'mount' becomes 'stage' (I've seen this before).
'tailless' means to remove the last letter.
'stage' with its final letter taken away is 'STAG'.
(Other definitions for stag that I've seen before include "Game - speculator" , "Hunted animal" , "This party is for men only" , "Investor" , "One who speculates" .)