Four employ five climbing US volcano (8)
I believe the answer is:
vesuvius
'volcano' is the definition.
(I've seen this before)
'four employ five climbing us' is the wordplay.
'four' becomes 'iv' (roman numeral).
'employ' becomes 'use' ('use' can be a synonym of 'employ'**).
'five' becomes 'v' (roman numeral).
'climbing' shows that the letters should be reversed in order (in a down clue, letters climb up).
'iv'+'use'+'v'='ivusev'
'ivusev' reversed gives 'vesuvi'.
'vesuvi'+'us'='VESUVIUS'
(Other definitions for vesuvius that I've seen before include "Pompeii volcano" , "Volcano near Naples" , "Italian volcano" , "It destroyed Pompeii" .)