Dark web protected by monster (8)
I believe the answer is:
brunette
'dark' is the definition.
(similar in meaning)
'web protected by monster' is the wordplay.
'web' becomes 'net' (synonyms).
'protected by' indicates putting letters inside (inserted letters are held or protected).
'monster' becomes 'brute' (I've seen this in another clue).
'net' placed within 'brute' is 'BRUNETTE'.
(Other definitions for brunette that I've seen before include "Kind of dark girl" , "She's certainly not a blonde" , "One having dark brown hair" , "Not what gentlemen would choose" , "Female with dark brown hair" .)