Mighty river shelter (6)
I believe the answer is:
potent
'mighty' is the definition.
(I've seen this before)
'river shelter' is the wordplay.
'river' becomes 'Po' (Italian river).
'shelter' becomes 'tent' (canvas shelter).
'po'+'tent'='POTENT'
(Other definitions for potent that I've seen before include "Strong or effective" , "Mighty" , "Intoxicating" , "Strong or concentrated, like some drink" , "Concentrated or strong, like liquor" .)