Wrestling overtly at first with problem (4)
I believe the answer is:
sumo
'wrestling' is the definition.
(I've seen this before)
'overtly at first with problem' is the wordplay.
'at first' suggests taking the first letters.
'with' means one lot of letters go next to another.
'problem' becomes 'sum' (arithmetic problem).
The first letter of 'overtly' is 'o'.
'o' after 'sum' is 'SUMO'.
(Other definitions for sumo that I've seen before include "Japanese wrestling for huge men" , "Japanese wrestling sport of over-weight men" , "Type of Japanese wrestling" , "Traditional Japanese wrestling by fat men" , "foreign form of fighting" .)