A nut with business at Birkenhead (3)
I believe the answer is:
cob
'a nut' is the definition.
(I know that cob is a type of edible nut)
'business at birkenhead' is the wordplay.
'business' becomes 'co' (short for company).
'at' is a charade indicator (letters next to each other).
'birkenhead' becomes 'b' (head letter of 'birken').
'co'+'b'='COB'
'with' acts as a link.
(Other definitions for cob that I've seen before include "mount" , "Hazelnut" , "Source of bread" , "Swan of Jacobean origin" , "mixture of clay and straw" .)