Where location of Portsmouth harbour entrance is irrelevant (6,3,5)
I believe the answer is:
beside the point
'irrelevant' is the definition.
(I've seen this before)
'where location of portsmouth harbour entrance is' is the wordplay.
I cannot quite see how this works, but
'location' could be 'point' (point is a kind of location) and 'point' is located in the answer.
'entrance' could be 'side' (side entrance is a kind of entrance) and 'side' is present in the answer.
'is' could be 'be' and 'be' is present in the answer.
The remaining letters 'the' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for beside the point that I've seen before include "It's immaterial" , "as the end of sentence?" , "Irrelevant" , "Sentence starts or ends here" .)