British leading American military survey in part of Wales (6)
I believe the answer is:
brecon
'part of wales' is the definition.
'british leading american military survey' is the wordplay.
'leading' indicates taking the first letters.
'american military survey' becomes 'recon' (short for reconnaissance).
The first letter of 'british' is 'b'.
'b'+'recon'='BRECON'
'in' acts as a link.
(Other definitions for brecon that I've seen before include "- Beacons, mountains in Wales" , "Market town" , "Town in Mid-Wales" , "Welsh town (with beacons)" , "tourist in Wales might come to" .)