Mighty Italian river shelter (6)
I believe the answer is:
potent
'mighty' is the definition.
(I've seen this in another clue)
'italian river shelter' is the wordplay.
'italian river' becomes 'po' (river in Italy).
'shelter' becomes 'tent' (canvas shelter).
'po'+'tent'='POTENT'
(Other definitions for potent that I've seen before include "Powerful, influential" , "Intoxicating" , "Top Ten (anag.)" , "Strong in operation" , "Forceful" .)