River containing 75 per cent of fish, lower (6)
I believe the answer is:
debase
'fish lower' is the definition.
'debase' can be an answer for 'lower' (I've seen this before). I am not certain of the 'fish' bit.
'river containing 75 per cent' is the wordplay.
'river' becomes 'Dee' (River Dee).
'containing' means one lot of letters goes inside another.
'75 per cent' becomes 'bas' (I can't justify this - if you can you should give a lot more credence to this answer).
'dee' going around 'bas' is 'DEBASE'.
'of' acts as a link.
Can you help me to learn more?
(Other definitions for debase that I've seen before include "Degrade or demean" , "Lower -- seabed (anag)" , "Make lower in prestige or esteem" , "dishonour" , "Degrade or devalue" .)