Backing European missile business (5)
I believe the answer is:
trade
'business' is the definition.
(I know that trade is a type of business)
'backing european missile' is the wordplay.
'backing' says the letters should be written backwards.
'european' becomes 'E' (abbreviation e.g. EU).
'missile' becomes 'dart' (Backwards).
'e'+'dart'='edart'
'edart' backwards is 'TRADE'.
(Other definitions for trade that I've seen before include "Calling" , "Barter, do business" , "Rated (anag.)" , "Buy and sell in business" , "Deal - occupation" .)