Only about five work out (5)
I believe the answer is:
solve
'work out' is the definition.
(thesaurus)
'only about five' is the wordplay.
'only' becomes 'sole' ('sole' can be a synonym of 'only').
'about' means one lot of letters goes inside another.
'five' becomes 'v' (roman numeral).
'sole' enclosing 'v' is 'SOLVE'.
(Other definitions for solve that I've seen before include "Find the solution to puzzle or problem" , "Find the 3dn to a mystery" , "Unscramble" , "Deal with (a problem)" , "Find the answer to this clue, for instance" .)