Monsters of ghastly reputation, enormous size originally (5)
I believe the answer is:
ogres
'monsters' is the definition.
('ogre' can be a synonym of 'monster')
'of ghastly reputation enormous size originally' is the wordplay.
'originally' says to take the initial letters.
The first letters of 'of ghastly reputation enormous size' is 'OGRES'.
(Other definitions for ogres that I've seen before include "Formidable people" , "huge figures" , "Gorse (anag.)" , "Awful brutes" , "Beasts" .)