Possibly score a hundred in football game (6)
I believe the answer is:
soccer
'football game' is the definition.
(I know that soccer is a type of football game)
'possibly score a hundred in' is the wordplay.
'possibly' is an anagram indicator.
'a hundred' becomes 'C' (Roman numeral).
'in' means one lot of letters goes inside another.
'score' is an anagram of 'socer'.
'socer' placed around 'c' is 'SOCCER'.
(Other definitions for soccer that I've seen before include "Ball game" , "popular game" , "Association football" , "Roy Keane's game" , "Informally, game" .)