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