Puts back acquiring European organisation (5)
I believe the answer is:
setup
'organisation' is the definition.
(I've seen this before)
'puts back acquiring european' is the wordplay.
'back' shows that the letters should be reversed in order.
'acquiring' means one lot of letters goes inside another.
'european' becomes 'E' (abbreviation e.g. EU).
'puts' backwards is 'stup'.
'stup' enclosing 'e' is 'SETUP'.
(Other definitions for setup that I've seen before include "Organisation (informal)" , "Fix" .)