A run on first class return causes strain (4)
I believe the answer is:
aria
'strain' is the definition.
(aria is a kind of strain)
'a run on first class return' is the wordplay.
'a run' becomes 'ar' (I've seen this in other clues**).
'on' is a charade indicator (letters next to each other).
'first class' becomes 'ai' (Aa - top quality).
'return' says the letters should be written backwards.
'ai' backwards is 'ia'.
'ar'+'ia'='ARIA'
'causes' is the link.
(Other definitions for aria that I've seen before include "Operatic solo, duet, etc" , "Song, one from 'West Side Story' needing no introduction" , "Operatic tune" , "Opera solo" , "Type of song" .)