Fish seller hooking customer, finally (5)
I believe the answer is:
trout
'fish' is the definition.
(I know that trout is a type of fish)
'seller hooking customer finally' is the wordplay.
'seller' becomes 'tout' (tout is a kind of seller).
'hooking' is an insertion indicator (some letters are hooked inside).
'finally' suggests the final letters.
The final letter of 'customer' is 'r'.
'tout' enclosing 'r' is 'TROUT'.
(Other definitions for trout that I've seen before include "Edible fish" , "Rainbow, for example" , "Catch" , "Mistiness" , "Tutor (anag.)" .)