The coach seems a bighead to us (3)
I believe the answer is:
bus
'the coach' is the definition.
(synonyms)
'a bighead to us' is the wordplay.
'a bighead' becomes 'b' (the head letter of 'big').
'b'+'us'='BUS'
'seems' acts as a link.
(Other definitions for bus that I've seen before include "Coach" , "People carrier" , "eg Double-decker" , "Something caught" , "Means of public transport" .)