Setter maybe returning with post is a stroke of luck (7)
I believe the answer is:
godsend
'a stroke of luck' is the definition.
(I've seen this before)
'setter maybe returning with post' is the wordplay.
'setter maybe' becomes 'dog' (setter is a type of dog).
'returning' says the letters should be written backwards.
'with' says to put letters next to each other.
'post' becomes 'send' (synonyms).
'dog' in reverse letter order is 'god'.
'god'+'send'='GODSEND'
'is' acts as a link.
(Other definitions for godsend that I've seen before include "Very helpful thing" , "The relief" , "Very welcome thing" , "Lucky find" , "Welcome good fortune" .)