Go off seat that's lost its top (5)
I believe the answer is:
addle
'go off' is the definition.
(eg an egg might addle)
'seat that's lost its top' is the wordplay.
'seat' becomes 'saddle' (saddle is a kind of seat).
'that's lost its top' means to remove the first letter.
'saddle' with its first letter taken off is 'ADDLE'.
(Other definitions for addle that I've seen before include "Befuddle" , "Confuse; go off" , "Make confused - go bad" , "Scramble" , "Mix up or confuse" .)