Talk excessively about flop (3)
I believe the answer is:
sag
'flop' is the definition.
(I've seen this in another clue)
'talk excessively about' is the wordplay.
'talk excessively' becomes 'gas' (I've seen this before).
'about' shows that the letters should be reversed in order.
'gas' backwards is 'SAG'.
(Other definitions for sag that I've seen before include "Subsidence" , "Cave in" , "Slump" , "Droop in the middle - it's gas" , "Droop, flag" .)