What’s key in Old English poem (3)
I believe the answer is:
ode
'poem' is the definition.
(ode is a kind of poem)
'what's key in old english' is the wordplay.
'what's key' becomes 'd' (I've seen this in other clues).
'in' indicates putting letters inside.
'old english' becomes 'oe'.
'd' placed inside 'oe' is 'ODE'.
(Other definitions for ode that I've seen before include "Poem in praise of doe" , "Lyric poem" , "Verse form" , "- - to a 12 Urn" , "Sung poem" .)