Last monarch turned back by chief (6)
I believe the answer is:
remain
'last' is the definition.
(I've seen this before)
'monarch turned back by chief' is the wordplay.
'monarch' becomes 'er' (abbreviation for Elizabeth Regina).
'turned back' says the letters should be written in reverse.
'by' says to put letters next to each other.
'chief' becomes 'main' (I've seen this before**).
'er' backwards is 're'.
're'+'main'='REMAIN'
(Other definitions for remain that I've seen before include "Survive" , "Stay on after others have gone" , "Stay (in position)" , "Stay behind" , "stop behind?" .)