Hybrid with massive head and longer form (7)
I believe the answer is:
mongrel
'hybrid' is the definition.
(I've seen this before)
'massive head and longer form' is the wordplay.
'head' says to take the initial letters.
'and' says to put letters next to each other.
'form' indicates anagramming the letters (a different form of the same letters).
The initial letter of 'massive' is 'm'.
'longer' is an anagram of 'ongrel'.
'm'+'ongrel'='MONGREL'
'with' is the link.
(Other definitions for mongrel that I've seen before include "Mixed-breed dog" , "Dog of no recognisable breed" , "mixed stock" , "Cross" , "Non-pedigree" .)