Barge constructed by firm in the West Country (4)
I believe the answer is:
scow
'barge' is the definition.
(I know that scow is a type of barge)
'firm in the west country' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'in the' indicates putting letters inside.
'west country' becomes 'SW' (area of the South West of England).
'co' put within 'sw' is 'SCOW'.
'constructed by' is the link.
(Other definitions for scow that I've seen before include "ighter" , "Wide-beamed dinghy" , "Low boat" , "Flat-bottomed boat for cows" , "Maybe lighter" .)