Old pack leader’s forehead now large (5,3)
I believe the answer is:
brown owl
'old pack leader's' is the definition.
(name for the leader in a Brownie pack)
'forehead now large' is the wordplay.
'forehead' becomes 'brow' ('brow' can be a synonym of 'forehead').
'large' becomes 'l'.
'brow'+'now'+'l'='BROWN OWL'
(Other definitions for brown owl that I've seen before include "young leader" , "Guide who leads" , "Bird" .)