Duke appearing in Old English poem (3)
I believe the answer is:
ode
'poem' is the definition.
(ode is a type of poem)
'duke appearing in old english' is the wordplay.
'duke' becomes 'd' (abbreviation).
'appearing in' is an insertion indicator.
'old english' becomes 'OE'.
'd' placed within 'oe' is 'ODE'.
(Other definitions for ode that I've seen before include "Poetic composition" , "poetry" , "- - to a 12 Urn" , "Verse form" , "Literary work" .)