Wear down queen with poem (5)
I believe the answer is:
erode
'wear down' is the definition.
(to erode is to wear down or decay)
'queen with poem' is the wordplay.
'queen' becomes 'ER' (short for Elizabeth Regina ie Queen Elizabeth).
'with' is a charade indicator (letters next to each other).
'poem' becomes 'ode' (ode is a type of poem).
'er'+'ode'='ERODE'
(Other definitions for erode that I've seen before include "Eat away" , "Abrade" , "Gradually wear away" , "go to pieces!" , "Become ground down or deteriorate" .)