Mount very retrograde operation against one from America (8)
I believe the answer is:
vesuvius
'mount' is the definition.
(Mount Vesuvius)
'very retrograde operation against one from america' is the wordplay.
'very' becomes 'v' (abbreviation).
'retrograde' shows that the letters should be reversed in order.
'operation' becomes 'use'.
'against' becomes 'vi' (I can't justify this - if you can you should believe this answer much more).
'one from america' becomes 'us'.
'use' back-to-front is 'esu'.
'v'+'esu'+'vi'+'us'='VESUVIUS'
(Other definitions for vesuvius that I've seen before include "Pompeii volcano" , "Italian volcano" , "Volcano near Naples" , "It destroyed Pompeii" .)