Labour losing Welsh constituency (4)
I believe the answer is:
seat
'constituency' is the definition.
(House of Commons seats represent constituencies)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for seat that I've seen before include "Position (in parliament)" , "Box ---, position of advantage" , "Constituency; location" , "Chair, bench, saddle, etc" , "Parliamentary constituency" .)