Climbing record broken by unknown resident abroad (5)
I believe the answer is:
expat
'resident abroad' is the definition.
(I've seen this before)
'climbing record broken by unknown' is the wordplay.
'climbing' says the letters should be written backwards (in a down clue, letters climb up).
'record' becomes 'tape' (taping record is a kind of recording).
'broken by' is an insertion indicator (inserted letters break into the word).
'unknown' becomes 'x' (algebra).
'tape' enclosing 'x' is 'tapxe'.
'tapxe' back-to-front is 'EXPAT'.
(Other definitions for expat that I've seen before include "one settled abroad" , "one working abroad" , "One living in foreign country" , "who now uses overseas address?" , "he has emigrated" .)