Class ace getting into new structure (9)
I believe the answer is:
formation
'structure' is the definition.
(I've seen this before)
'class ace getting into new' is the wordplay.
'class' becomes 'form' (form class is a kind of class**).
'ace' becomes 'a'.
'getting' means one lot of letters go next to another.
'new' is an anagram indicator.
'into' is an anagram of 'tion'.
'form'+'a'+'tion'='FORMATION'
(Other definitions for formation that I've seen before include "Institution" , "Arrangement of things acting as a unit" , "Establishment" , "Genesis" , "Composition" .)