Post mountain-climbing setter in building (9)
I believe the answer is:
placement
'post' is the definition.
(both can mean a job)
'mountain-climbing setter in building' is the wordplay.
'mountain' becomes 'alp' (the Alps are a mountain range).
'climbing' says the letters should be written in reverse (in a down clue, letters climb up).
'setter in building' becomes 'cement' (substance which sets used in construction).
'alp' backwards is 'pla'.
'pla'+'cement'='PLACEMENT'
(Other definitions for placement that I've seen before include "Temporary job for work experience" , "period of work experience" , "Arrangement" , "Temporary posting" , "location" .)