Companion for dam built at last in south of France (4)
I believe the answer is:
stud
'companion for dam' is the definition.
'built at last in south of france' is the wordplay.
'at last' indicates one should take the final letters.
'in' is an insertion indicator.
'south of france' becomes 'sud' (French for south).
The final letter of 'built' is 't'.
't' going within 'sud' is 'STUD'.
(Other definitions for stud that I've seen before include "Press fastener" , "card game" , "Collar fastener or horse farm" , "Football boot's peg-like projection" , "Variety of poker" .)