Iron was outside quarters being decorated (9)
I believe the answer is:
festooned
'being decorated' is the definition.
(I've seen this in another clue)
'iron was outside quarters' is the wordplay.
'iron' becomes 'Fe' (chemical symbol for iron).
'was' becomes 'stood' (I can't justify this - if you can you should believe this answer much more).
'outside' is an insertion indicator (some letters go outside others).
'quarters' becomes 'ne' (N and E are quarters on a compass).
'stood' enclosing 'ne' is 'stooned'.
'fe'+'stooned'='FESTOONED'
Can you help me to learn more?
(Another definition for festooned that I've seen is " Adorned with garlands".)