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