Playing well, Arsenal's wingers relaxed (8)
I believe the answer is:
'relaxed' is the definition.
(informal can mean relaxed or casual)
'playing well arsenal's wingers' is the wordplay.
'playing well' becomes 'in form' (to be 'in form' is to be at one's best in a sport).
'wingers' means to remove the middle letters (letters on the edges or wings of the word).
'arsenal' with its middle taken out is 'al'.
(Other definitions for informal that I've seen before include "Cosy -- loose" , "Casual and unceremonious" , "Unofficial, relaxed" , "Casual, not official" , "If normal, be casual and without ceremony" .)