Remove monarch on wild sea (5)
I believe the answer is:
erase
'remove' is the definition.
(I've seen this before)
'monarch on wild sea' is the wordplay.
'monarch' becomes 'er' (abbreviation for Elizabeth Regina, British Queen).
'on' is an insertion indicator.
'wild' indicates anagramming the letters.
'sea' anagrammed gives 'ase'.
'er' placed into 'ase' is 'ERASE'.
(Other definitions for erase that I've seen before include "Remove all traces of" , "Delete" , "Scratch" , "Blot out" , "Remove evidence of" .)