Reject taking it back for invalid (4,2)
I believe the answer is:
shut in
'invalid' is the definition.
(I know that invalid can be written as shut-in)
'reject taking it back' is the wordplay.
'reject' becomes 'shun' (I can't justify this - if you can you should give a lot more credence to this answer).
'taking' is an insertion indicator.
'back' says the letters should be written backwards.
'it' written backwards gives 'ti'.
'shun' enclosing 'ti' is 'SHUT-IN'.
'for' acts as a link.
Can you help me to learn more?
(Another definition for shut in that I've seen is " Enclosed".)