Concede Her Majesty must retire in time (5)
I believe the answer is:
agree
'concede' is the definition.
(thesaurus)
'her majesty must retire in time' is the wordplay.
'her majesty' becomes 'er' (Elizabeth Regina).
'must retire' is a reversal indicator.
'in' means one lot of letters goes inside another.
'time' becomes 'age' (age is a kind of time).
'er' written backwards gives 're'.
're' placed inside 'age' is 'AGREE'.
(Other definitions for agree that I've seen before include "Say yes to a suggestion" , "confess?" , "Concur - correspond" , "Arrive at a settlement" , "Go along with" .)