I'm surprised to receive back payment for strain (6)
I believe the answer is:
melody
'for strain' is the definition.
(synonyms)
'i'm surprised to receive back payment' is the wordplay.
'i'm surprised' becomes 'my' ('my' is an expression of surprise).
'to receive' means one lot of letters goes inside another.
'back' says the letters should be written backwards.
'payment' becomes 'dole' (dole can mean government benefits).
'dole' in reverse letter order is 'elod'.
'my' enclosing 'elod' is 'MELODY'.
(Other definitions for melody that I've seen before include "Strain" , "My dole (anag.)" , "Air, tune" , "It forms the core of a tune" , "A pleasant tune or song" .)