No official American centre for koalas (7)
I believe the answer is:
refusal
'no' is the definition.
(refusing is saying no)
'official american centre for koalas' is the wordplay.
'official' becomes 'ref' (match official in football, say).
'american' becomes 'us' (abbreviation for United States).
'centre for' says to take the centre.
The middle of 'koalas' is 'al'.
'ref'+'us'+'al'='REFUSAL'
(Other definitions for refusal that I've seen before include "Rebuff" , "Instance of a horse not jumping" , "The turning down of a request" , "fault in the show ring" , "(Horse) failure to jump" .)