Sea-monster in Majorca (3)
I believe the answer is:
orc
'sea-monster' is the definition.
(I've seen this before)
'in majorca' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ORC' is hidden within 'majorca'.
(Other definitions for orc that I've seen before include "Tolkien ogre" , "Killer whale" , "Tolkien creature" , "warlike figure" , "Imaginary creature" .)