Most relaxed game set outside school (7)
I believe the answer is:
loosest
'most relaxed' is the definition.
(I've seen this in another clue)
'game set outside school' is the wordplay.
'game' becomes 'loo' (historic card game found in Jane Austen novels).
'outside' is an insertion indicator.
'school' becomes 'S'.
'set' enclosing 's' is 'sest'.
'loo'+'sest'='LOOSEST'
(Other definitions for loosest that I've seen before include "most unstable" , "most relaxed" .)