The top's come off the seat? Rot! (5)
I believe the answer is:
addle
'rot' is the definition.
(I've seen this before)
'the top's come off the seat?' is the wordplay.
'the top's come off' means to remove the first letter.
'the seat?' becomes 'saddle' (saddle is a kind of seat).
'saddle' with its initial letter removed is 'ADDLE'.
(Other definitions for addle that I've seen before include "Rot" , "Confuse; become rotten" , "becomes muddled" , "(Of eggs) turn bad" , "Make confused - go bad" .)