Swim with right weed (4)
I believe the answer is:
drip
'weed' is the definition.
(thesaurus)
'swim with right' is the wordplay.
'swim' becomes 'dip' (dip can mean a brief swim).
'with' is an insertion indicator.
'right' becomes 'r' (common abbreviation).
'dip' enclosing 'r' is 'DRIP'.
(Other definitions for drip that I've seen before include "Fall in small droplets as from tap" , "Wet - sign of faulty washer" , "Fall from a leaking tap - weakling" , "Means of intravenous feeding" , "Weak and ineffectual person" .)