Tough cut of meat — that's by the way (4,8)
I believe the answer is:
hard shoulder
'that's by the way' is the definition.
(area to the side of a road or way)
'tough cut of meat' is the wordplay.
'tough' becomes 'hard' (synonyms).
'cut of meat' becomes 'shoulder' (shoulder is a kind of cut of meat).
'hard'+'shoulder'='HARD SHOULDER'
(Other definitions for hard shoulder that I've seen before include "emergency situation" , "space for emergency stops" , "Strip by a busy road" , "Part of motorway" , "Motorway breakdown lane" .)