Thaw? Opt to change route by river perhaps (7)
I believe the answer is:
towpath
'route by river perhaps' is the definition.
I can't judge whether this definition defines the answer.
'thaw? opt to change' is the wordplay.
'to change' indicates anagramming the letters.
'thaw'+'opt'='thawopt'
'thawopt' with letters rearranged gives 'TOWPATH'.
Can you help me to learn more?
(Other definitions for towpath that I've seen before include "Waterside track for horses" , "Canalside track" , "Canalside way" , "Way beside a canal" , "Track by waterway" .)