Duke in Old English poem (3)
I believe the answer is:
ode
'poem' is the definition.
(ode is a kind of poem)
'duke in old english' is the wordplay.
'duke' becomes 'd' (abbreviation).
'in' indicates putting letters inside.
'old english' becomes 'oe'.
'd' inserted within 'oe' is 'ODE'.
(Other definitions for ode that I've seen before include "poetry" , "Sung poem" , "Lines" , "Poem expressing noble feelings" , "A poem celebrating person or event" .)