Corporation tax unwarranted methinks, initially (3)
I believe the answer is:
tum
'corporation' is the definition.
(I've seen this before)
'tax unwarranted methinks initially' is the wordplay.
'initially' suggests taking the first letters.
The first letters of 'tax unwarranted methinks' is 'TUM'.
(Other definitions for tum that I've seen before include "Belly" , "Middle" , "Stomach (informal)" , "Corporation" .)