What you must do, only having five to fill in (5)
I believe the answer is:
solve
'what you must do' is the definition.
(the person reading the clue has to solve it)
'only having five to fill in' is the wordplay.
'only' becomes 'sole' (synonyms).
'having five' becomes 'v' (I am not sure about this - if you are sure you should believe this answer much more).
'to fill in' means one lot of letters goes inside another.
'sole' placed around 'v' is 'SOLVE'.
Can you help me to learn more?
(Other definitions for solve that I've seen before include "as you're trying to do now" , "Unravel (a clue)" , "Get the answers" , "do crosswords" , "Work out answer to puzzle" .)