Seaweed that is being found in Scottish waters? (6)
I believe the answer is:
kelpie
'being found in scottish waters?' is the definition.
(mythical Scottish water spirit)
'seaweed that is' is the wordplay.
'seaweed' becomes 'kelp' (I've seen this before).
'that is' becomes 'ie' (synonyms).
'kelp'+'ie'='KELPIE'
(Other definitions for kelpie that I've seen before include "A malevolent Scottish water spirit like a horse" , "Malevolent Scottish spirit in form of horse" , "A Scottish water-spirit in the shape of a horse" , "Scottish water sprite -- Australian sheepdog" .)