A nut in company with a bighead (3)
I believe the answer is:
cob
'a nut' is the definition.
(cob is a kind of nut)
'company with a bighead' is the wordplay.
'company' becomes 'co' (abbreviation).
'with' means one lot of letters go next to another.
'a bighead' becomes 'b' (the head letter of 'big').
'co'+'b'='COB'
'in' acts as a link.
(Other definitions for cob that I've seen before include "mixture of clay and straw" , "Hazelnut" , "Horse, male swan and loaf of bread" , "Round loaf of bread" , "Source of bread" .)