Head of menagerie, maestro trained monkey (8)
I believe the answer is:
marmoset
'monkey' is the definition.
(marmoset is a kind of monkey)
'head of menagerie maestro trained' is the wordplay.
'head of' says to take the initial letters.
'trained' indicates an anagram (to train can mean to grow something into a shape).
The initial letter of 'menagerie' is 'm'.
'maestro' anagrammed gives 'armoset'.
'm'+'armoset'='MARMOSET'
(Other definitions for marmoset that I've seen before include "Small bushy-tailed monkey" , "Somme rat? (anag)" , "Small South American monkey" , "Primate" , "Monkey with claws" .)