On the hard shoulder, 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)
'on the hard shoulder' is the wordplay.
'on' means one lot of letters goes inside another (as in clothing 'on' a person).
'hard shoulder' becomes 'byway' (I am not sure about this - if you are sure you should believe this answer much more).
'the' inserted into 'byway' is 'BY THE WAY'.
Can you help me to learn more?
(Other definitions for by the way that I've seen before include "In point of fact" , "In passing, while we're on the subject" , "Incidentally, as a rather irrelevant comment" , "Where 14's [HARD SHOULDER] situated" , "At the side of the path?" .)