Firm returning some gadget a nit's bought (9)
I believe the answer is:
obstinate
'firm' is the definition.
(I've seen this before)
'returning some gadget a nit's bought' is the wordplay.
'returning' says the letters should be written backwards.
'some' indicates a hidden word (some of the parts of the word are used).
'etanitsbo' is hidden within 'gadget a nits bought'.
'etanitsbo' reversed gives 'OBSTINATE'.
(Other definitions for obstinate that I've seen before include "Cussed" , "Stubborn or unyielding" , "Firm" , "Female bird" , "such as a mule" .)