No longer the thing to make an issue out of (4)
I believe the answer is:
exit
'out of' is the definition.
'exit' can be an answer for 'out' (I've seen this in another clue). I am unsure of the 'of' bit.
'no longer the thing to make an' is the wordplay.
I cannot quite understand how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is present in the answer.
'an' could be 'i' (Roman numeral for one) and 'i' is found in the answer.
'no longer' could be 'ex' (prefix meaning former or onetime) and 'ex' is located in the answer.
No letters remain.
This may be the basis of clue (or it may be nonsense).
'issue' acts as a link.
Can you help me to learn more?
(Other definitions for exit that I've seen before include "Act of going out" , "Depart" , "farewell!" , "Escape route?" , "[DOOR] ?" .)