Sent a snap to be developed, incidentally (2,7)
I believe the answer is:
en passant
'incidentally' is the definition.
(thesaurus)
'sent a snap to be developed' is the wordplay.
'to be developed' indicates anagramming the letters.
'sent'+'a'+'snap'='sentasnap'
'sentasnap' anagrammed gives 'EN PASSANT'.
(Other definitions for en passant that I've seen before include "In passing, by the way from French" , "An aptness, by the way, for chess move" , "Incidentally, in the course of doing something else" , "while going by" , "Term used on board" .)