Join the session when there's something wrong about it (3,2)
I believe the answer is:
sit in
'join the session' is the definition.
'sit in' can be an answer for 'join' (thesaurus). I'm not certain of the 'the session' bit.
'something wrong about it' is the wordplay.
'something wrong' becomes 'sin' (I've seen this before).
'about' is an insertion indicator.
'sin' enclosing 'it' is 'SIT-IN'.
'when there's' acts as a link.
Can you help me to learn more?
(Other definitions for sit in that I've seen before include "Act as a substitute for" , "act as someone's deputy" , "demonstration" , "form of protest" .)