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' says to put letters next to each other.
'pleasure' becomes 'fun' (fun is enjoyment or pleasure).
'in'+'fun'='IN FUN'
'yet' is the link.
(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?" .)