Take advantage of the river's being in semi-flood (6)
I believe the answer is:
impose
'take advantage' is the definition.
(I've seen this in another clue)
'of the river's being in semi-flood' is the wordplay.
I cannot quite understand how this works, but
'of' could be 'o' and 'o' is found within the answer.
'being' could be 'imp' (imp is a kind of being) and 'imp' is present in the answer.
The remaining letters 'se' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for impose that I've seen before include "Decree" , "Put upon, say a tax" , "Force (something) to be done or accepted" , "As task, set" , "Inflict (something unpleasant)" .)