Strong riverside canvas shelter (6)
I believe the answer is:
potent
'strong' is the definition.
(I know that strong can be written as potent)
'riverside canvas shelter' is the wordplay.
'riverside' becomes 'po' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'canvas shelter' becomes 'tent' (I've seen this in another clue).
'po'+'tent'='POTENT'
Can you help me to learn more?
(Other definitions for potent that I've seen before include "Powerful like strong drink" , "Strong or concentrated, like some drink" , "Maybe overpowering" , "Mighty" , "Forceful" .)