Like most others, I have finished except for 99 and am not happy that I have a good solution. What if we split the clue as -
Hold on to catch/when suspended over the river.
ie build up the answer as "when" or a similar word (as, while, etc) surrounding a short word for a river - perhaps a specific river, or a general term, or "r".
Alternatively, it could be a synonym for suspended (eg hung) in front of, or surrounding, the river bit.
Catch can mean many things - sporting (as in cricket), fishing, snag or problem, bolt or lock, etc.
Lots of questions but no answer, I'm afraid.