Refuge in the middle of the garden retreat (7)
I believe the answer is:
harbour
'refuge' is the definition.
(I know that harbour is a type of refuge)
'middle of the garden retreat' is the wordplay.
'middle of the garden' becomes 'h' (I can't justify this - if you can you should believe this answer much more).
'retreat' becomes 'arbour' (I've seen this before).
'h'+'arbour'='HARBOUR'
'in the' is the link.
Can you help me to learn more?
(Other definitions for harbour that I've seen before include "Keep in mind; refuge" , "Sheltered anchorage" , "Nurse" , "Ship haven or shelter" , "Sheltered port" .)