Mythical creature that is alongside headland (6)
I believe the answer is:
nessie
'mythical creature' is the definition.
(Nessie is an example)
'that is alongside headland' is the wordplay.
'that is' becomes 'ie' (abbreviation for id est, 'that is' in Latin).
'alongside' says to put letters next to each other.
'headland' becomes 'ness' (both can mean a promontory).
'ie' put after 'ness' is 'NESSIE'.
(Other definitions for nessie that I've seen before include "Scots loch monster (colloq.)" , "Nickname for a Scottish monster" , "Supposed loch monster" .)