Wash second baby (3,2)
I believe the answer is:
mop up
'wash' is the definition.
(thesaurus)
'second baby' is the wordplay.
'second' becomes 'mo' (both are short periods of time).
'baby' becomes 'pup'.
'mo'+'pup'='MOP-UP'
(Other definitions for mop up that I've seen before include "Finish a task, clear away remnants" , "take care of" , "Clear away, dispose of" , "Complete final stage (of a task)" , "Swab" .)