Not quite south, but that's irrelevant (6,3,5)
I believe the answer is:
beside the point
'that's irrelevant' is the definition.
(I've seen this before)
'not quite south' is the wordplay.
I cannot quite understand how this works, but
'not' could be 'nt' (synonyms) and 'nt' is located in the answer.
'south' could be 's' (abbreviation) and 's' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
'but' is the link.
Can you help me to learn more?
(Other definitions for beside the point that I've seen before include "Sentence starts or ends here" , "It's immaterial" , "as the end of sentence?" , "Irrelevant" .)