Where the units and tenths go is irrelevant (6,3,5)
I believe the answer is:
beside the point
'irrelevant' is the definition.
(I've seen this before)
'where the units and tenths go is' is the wordplay.
I cannot quite see how this works, but
an anagram of 'the' is 'eth' which is present in the answer.
'and' could be 'n' (common abbreviation for 'and') and 'n' is found within the answer.
'is' could be 'be' and 'be' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for beside the point that I've seen before include "Irrelevant" , "It's immaterial" , "as the end of sentence?" , "Sentence starts or ends here" .)