Monster’s head, that is (6)
I believe the answer is:
nessie
'monster's' is the definition.
(Nessie is an example)
'head that is' is the wordplay.
'head' becomes 'ness' (ness is a headland).
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'ness'+'ie'='NESSIE'
(Other definitions for nessie that I've seen before include "Nickname for a Scottish monster" , "Scots loch monster (colloq.)" , "Supposed loch monster" .)