Bit of a simpleton, you'll allow (3)
I believe the answer is:
let
'allow' is the definition.
(I know that allow can be written as let)
'bit of a simpleton you'll' is the wordplay.
'bit of' indicates the answer is hidden within the clue.
'LET' can be found hidden inside 'simpleton youll'.
(Other definitions for let that I've seen before include "hired" , "Rent out (property)" , "Allow; leased" , "Allow - hire" , "& 15 Triple (anag.)" .)