Currently playing well, Arsenal's wingers relaxed (8)
I believe the answer is:
informal
'relaxed' is the definition.
(informal can mean relaxed or casual)
'currently playing well arsenal's wingers' is the wordplay.
'currently' becomes 'in' (I can't justify this - if you can you should give a lot more credence to this answer).
'playing well' becomes 'form' (as in a sportsperson who is playing 'on form').
'wingers' says to hollow out the word (remove centre letters) (letters on the edges or wings of the word).
'arsenal' with its middle taken out is 'al'.
'in'+'form'+'al'='INFORMAL'
Can you help me to learn more?
(Other definitions for informal that I've seen before include "If normal, be casual and without ceremony" , "Casual, free and easy" , "Casual, not official" , "Cosy -- loose" , "Casual or unofficial" .)