Four utilise five climbing American volcano (8)
I believe the answer is:
vesuvius
'volcano' is the definition.
(I've seen this before)
'four utilise five climbing american' is the wordplay.
'four' becomes 'iv' (roman numeral).
'utilise' becomes 'use' ('use' can be a synonym of 'utilize').
'five' becomes 'v' (roman numeral).
'climbing' says the letters should be written in reverse (in a down clue, letters climb up).
'american' becomes 'us'.
'iv'+'use'+'v'='ivusev'
'ivusev' back-to-front is 'vesuvi'.
'vesuvi'+'us'='VESUVIUS'
(Other definitions for vesuvius that I've seen before include "Pompeii volcano" , "It destroyed Pompeii" , "Volcano near Naples" , "Italian volcano" .)