Only about five crack (5)
I believe the answer is:
solve
'crack' 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' ('V' can be a synonym of 'five').
'sole' placed around 'v' is 'SOLVE'.
(Other definitions for solve that I've seen before include "Find the answer to a puzzle" , "Unravel (a clue)" , "Get the answers" , "Loves (anag) - to find an answer" , "Deal with (a problem)" .)