Absorbent canvas opens up on middle of river (9)
I believe the answer is:
retentive
'absorbent' is the definition.
Both the definition and answer are adjectives. Perhaps there's a link between them I don't understand?
'canvas opens up on middle of river' is the wordplay.
'canvas' becomes 'tent' (I've seen this in other clues).
'opens' indicates putting letters inside (some letters open a gap in others).
'up on middle of river' becomes 'reive' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'tent' put within 'reive' is 'RETENTIVE'.
Can you help me to learn more?
(Other definitions for retentive that I've seen before include "(Of memory) effective in holding information" , "Able to store memories etc" , "(Of mind) able to remember" , "must be able not to leak it maybe!" , "Readily recalling" .)