What one discusses is tailless antelope (4)
I believe the answer is:
topi
'antelope' is the definition.
(topi is a kind of antelope)
'what one discusses is tailless' is the wordplay.
'what one discusses' becomes 'topic' (topic of discussion).
'is tailless' means to remove the last letter (the tail of the word is removed).
'topic' with its last letter taken off is 'TOPI'.
(Other definitions for topi that I've seen before include "Tropical sun-hat" , "Large antelope; pith hat" , "Sun protection" , "Item of clothing" , "Pith-helmet" .)