Backing European Union about metal combine (5)
I believe the answer is:
unite
'combine' is the definition.
('unite' can be a synonym of 'combine')
'backing european union about metal' is the wordplay.
'backing' says the letters should be written in reverse.
'european union' becomes 'eu'.
'about' indicates putting letters inside.
'metal' becomes 'tin' (tin is a kind of metal**).
'eu' placed around 'tin' is 'etinu'.
'etinu' backwards is 'UNITE'.
(Other definitions for unite that I've seen before include "Come, or join together" , "fuse" , "Link" , "Bring together, become one" , "Join together, become allies" .)