Dog is lean after exercising: power in it (7)
I believe the answer is:
spaniel
'dog' is the definition.
(spaniel is a kind of dog)
'is lean after exercising power in it' is the wordplay.
'after exercising' is an anagram indicator.
'power' becomes 'p'.
'in it' is an insertion indicator.
'is'+'lean'='islean'
'islean' anagrammed gives 'saniel'.
'saniel' going around 'p' is 'SPANIEL'.
(Other definitions for spaniel that I've seen before include "Long-eared dog" , "Animal" , "Droopy-ear dog" , "Dog - leaps in (anag)" , "Kind of dog, may be a cocker" .)