Darling Nikki protects retreating monarch (4)
I believe the answer is:
king
'monarch' is the definition.
(male monarch)
'darling nikki protects retreating' is the wordplay.
'protects' indicates a hidden word (hidden letters are protected by the clue).
'retreating' says the letters should be written in reverse.
'gnik' is hidden in the letters of 'darling nikki'.
'gnik' backwards is 'KING'.
(Other definitions for king that I've seen before include "Male monarch" , "Male royal" , "One on throne" , "Sovereign" , "- Cobra; - crab; - post; - prawn" .)