Shout for river paddle (4)
I believe the answer is:
'shout' is the definition.
(roar is a kind of shout)
'river paddle' is the wordplay.
'river' becomes 'r' (abbreviation for river).
'paddle' becomes 'oar' (I've seen this in another clue).
'for' is the link.
(Other definitions for roar that I've seen before include "Zoom - prolonged cry" , "Yell" , "Bellow, holler" , "Shout loudly" , "Deep-throated cry" .)