Swim across river as fool (4)
I believe the answer is:
drip
'fool' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'swim across river' is the wordplay.
'swim' becomes 'dip' (dip can mean a brief swim).
'across' indicates putting letters inside.
'river' becomes 'r' (abbreviation for river).
'dip' enclosing 'r' is 'DRIP'.
'as' acts as a link.
Can you help me to learn more?
(Other definitions for drip that I've seen before include "Dribble" , "Fall in small droplets as from tap" , "Flow in small drops" , "Trickle -- weakling" , "Fall in small drops as from wet clothes" .)