Head that is becoming a monster? (6)
I believe the answer is:
nessie
'a monster?' is the definition.
(Nessie is an example)
'head that is' is the wordplay.
'head' becomes 'ness' (type of geographical point).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'ness'+'ie'='NESSIE'
'becoming' is the link.
(Other definitions for nessie that I've seen before include "Scots loch monster (colloq.)" , "Supposed loch monster" , "Nickname for a Scottish monster" .)