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