Kept quiet and all but closed garden building (7)

'kept quiet' is the definition.
Although both the answer and definition are past participle verbs, I don't understand how they can define each other.

'all but closed garden building' is the wordplay.
'all but' means to remove the last letter ('all but' can mean 'almost' or 'nearly').
'closed' becomes 'shut' (synonyms).
'garden building' becomes 'shed' (I've seen this before).
'shut' with its last letter removed is 'shu'.
'shu'+'shed'='SHUSHED'

'and' is the link.

Can you help me to learn more?