Best to follow leader of group (3,3)
I believe the answer is:
top dog
'leader of group' is the definition.
(top dog can mean a leader)
'best to follow' is the wordplay.
'best' becomes 'top' (top can mean the very best).
'to follow' becomes 'dog' (I've seen this before).
'top'+'dog'='TOP DOG'
(Other definitions for top dog that I've seen before include "One pre-eminent in their field" , "Leading light (informal)" , "Head honcho (at the kennels?)" , "Successful person - Crufts' winner?" , "Most important person (or winner at Crufts?)" .)