Climbing volunteers wear out clothing (6)
I believe the answer is:
attire
'clothing' is the definition.
(I know that clothing can be written as attire)
'climbing volunteers wear out' is the wordplay.
'climbing' is a reversal indicator (in a down clue, letters climb up).
'volunteers' becomes 'TA' (abbreviation for Territorial Army).
'wear out' becomes 'tire' (synonyms).
'ta' written backwards gives 'at'.
'at'+'tire'='ATTIRE'
(Other definitions for attire that I've seen before include "Clothes worn" , "Special clothing" , "Formal dress or costume" , "Clothing - tear it (anag)" , "Formal clothing" .)