500 appearing in Old English poem (3)
I believe the answer is:
ode
'poem' is the definition.
(I know that ode is a type of poem)
'500 appearing in old english' is the wordplay.
'500' becomes 'D' (Roman numeral for 500).
'appearing in' is an insertion indicator.
'old english' becomes 'oe'.
'd' put inside 'oe' is 'ODE'.
(Other definitions for ode that I've seen before include "Poem expressing noble feelings" , "Lyric poem" , "Lines" , "(Railway)system" , "Poetic composition" .)