It's beside path, incidentally (2,3,3)
I believe the answer is:
by the way
'incidentally' is the definition.
(I know that incidentally can be written as by the way)
'it's beside path' is the wordplay.
I cannot quite understand how this works, but
'beside' could be 'by' and 'by' is present in the answer.
'path' could be 'way' (way is a kind of path) and 'way' is found in the answer.
'it' could be 't' (abbreviation. e.g. in 'tis) and 't' is found in the answer.
The remaining letters 'he' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for by the way that I've seen before include "Where 14's [HARD SHOULDER] situated" , "In point of fact" , "At the roadside - incidentally" , "Incidentally, as a rather irrelevant comment" , "Where to find kerbstones" .)