Work out volume in one (5)
I believe the answer is:
solve
'work out' is the definition.
(thesaurus)
'volume in one' is the wordplay.
'volume' becomes 'v'.
'in' means one lot of letters goes inside another.
'one' becomes 'sole' ('sole' can be a synonym of 'one'**).
'v' placed inside 'sole' is 'SOLVE'.
(Other definitions for solve that I've seen before include "Find the answer to this clue, for instance" , "Unravel (a problem)" , "Work out answer to puzzle" , "Work out (a clue)" , "Find the 3dn to a mystery" .)