Head and tail of lamb, formerly (5)
I believe the answer is:
bonce
'head' is the definition.
(informal term for a person's head)
'tail of lamb formerly' is the wordplay.
'tail of' suggests the final letters.
'formerly' becomes 'once' (synonyms).
The final letter of 'lamb' is 'b'.
'b'+'once'='BONCE'
'and' acts as a link.
(Other definitions for bonce that I've seen before include "Noddle (slang)" , "Human head (informal)" , "Head (slang)" , "loaf" , "Nana" .)