In passing through the passage (2,3,3)
I believe the answer is:
by the way
'in passing' is the definition.
(I've seen this before)
'through the passage' is the wordplay.
'through' becomes 'by' ().
'passage' becomes 'way' (I've seen this in another clue).
'by'+'the'+'way'='BY THE WAY'
(Other definitions for by the way that I've seen before include "Where 14's [HARD SHOULDER] situated" , "In passing, while we're on the subject" , "Incidentally, as a rather irrelevant comment" , "At the side of the path?" , "Where to find kerbstones" .)