Strong post office shelter (6)
I believe the answer is:
potent
'strong' is the definition.
('potent' can be a synonym of 'strong')
'post office shelter' is the wordplay.
'post office' becomes 'po'.
'shelter' becomes 'tent' (tent is a kind of shelter**).
'po'+'tent'='POTENT'
(Other definitions for potent that I've seen before include "Powerful, influential" , "Having great power" , "Top Ten (anag.)" , "Concentrated or strong, like liquor" , "Strong or concentrated, like some drink" .)