Complain of pressure put on transport (4)
I believe the answer is:
carp
'complain' is the definition.
(thesaurus)
'pressure put on transport' is the wordplay.
'pressure' becomes 'p' (abbreviation).
'put on' means one lot of letters go next to another.
'transport' becomes 'car' (car is a kind of transport).
'p' after 'car' is 'CARP'.
'of' is the link.
(Other definitions for carp that I've seen before include "praise? The opposite" , "Raise petty objections about goldfish" , "Go on at" , "Complain continually" , "Fish - grouse" .)