Not out for pleasure, yet not serious (2,3)
I believe the answer is:
in fun
'not serious' is the definition.
'not out for pleasure' is the wordplay.
'not out' becomes 'in' ('in' is the opposite to 'out').
'for' is a charade indicator (letters next to each other).
'pleasure' becomes 'fun' (I've seen this before).
'in'+'fun'='IN FUN'
'yet' acts as a link.
I am not very happy about this link. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for in fun that I've seen before include "for a laugh" , "As a joke - not seriously" , "Not intended seriously" , "to raise a smile?" .)