On reflection, rushed argument's sound? (7)
I believe the answer is:
narrows
'sound?' is the definition.
(both are bodies of water)
'on reflection rushed argument's' is the wordplay.
'on reflection' is a reversal indicator.
'rushed' becomes 'ran'.
'argument's' becomes 'rows'.
'ran' in reverse letter order is 'nar'.
'nar'+'rows'='NARROWS'
(Other definitions for narrows that I've seen before include "Confines" , "Contracts" , "Reduces in width" , "Becomes less wide" , "Constricts" .)