Finish off second young seal (3,2)
I believe the answer is:
mop up
'finish off' is the definition.
('mop up' can be a synonym of 'finish off')
'second young seal' is the wordplay.
'second' becomes 'mo' (both are short periods of time).
'young seal' becomes 'pup' (I've seen this before).
'mo'+'pup'='MOP-UP'
(Other definitions for mop up that I've seen before include "take care of" , "Finish a task, clear away remnants" , "polish off" , "Finish off a task, clean away remnants" , "Kill (enemy stragglers)" .)