Practically monstrous old writer (4)
I believe the answer is:
hugo
'writer' is the definition.
(Victor Hugo)
'practically monstrous old' is the wordplay.
'practically' means to remove the last letter.
'monstrous' becomes 'huge'.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'huge' with its final letter taken off is 'hug'.
'hug'+'o'='HUGO'
(Other definitions for hugo that I've seen before include "Victor .... wrote ''Les Miserables''" , "Victor, say" , "Fr. Author; Sci-fi award" , "Jargon used by a particular group" , "French romantic writer, Victor, d. 1885" .)