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' (abbreviation for Elizabeth Regina).
'with' means one lot of letters go next to another.
'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" .)