Tolerate fat? Not initially (5)
I believe the answer is:
allow
'tolerate' is the definition.
(I've seen this before)
'fat? not initially' is the wordplay.
'fat?' becomes 'tallow' (I've seen this before).
'not initially' means to remove the first letter.
'tallow' with its initial letter removed is 'ALLOW'.
(Other definitions for allow that I've seen before include "OK" , "brook" , "praise no more" , "Sanction" , "Permit or concede" .)