In the river, Ed’s revealed as a weakling (5)
I believe the answer is:
weedy
'revealed as a weakling' is the definition.
I can't judge whether this definition defines the answer.
'in the river ed's' is the wordplay.
'in the' means one lot of letters goes inside another.
'river' becomes 'wey' (I can't justify this - if you can you should give a lot more credence to this answer).
'wey' going around 'ed' is 'WEEDY'.
Can you help me to learn more?
(Other definitions for weedy that I've seen before include "scrawny" , "Skinny" , "quite unlike Charles Atlas" , "Thin, puny (colloq.)" , "like an untended flower bed?" .)