Swim across river, feeble one! (4)
I believe the answer is:
drip
'feeble one' is the definition.
(term for a feeble person)
'swim across river' is the wordplay.
'swim' becomes 'dip' (dip is a kind of swim).
'across' is an insertion indicator.
'river' becomes 'r'.
'dip' going around 'r' is 'DRIP'.
(Other definitions for drip that I've seen before include "Weed - seep" , "Fall in small drops as from tap" , "Flow in small drops" , "See 6" , "Small drop; ineffectual person" .)