Second baby, clean (3,2)
I believe the answer is:
mop up
'clean' is the definition.
(to mop something up is to clean it)
'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 "take care of" , "Kill (enemy stragglers)" , "Clear away, dispose of" , "Finish off a task, clean away remnants" , "Complete final stage (of a task)" .)