I put the end of Endymion in the middle of poem (3)
I believe the answer is:
one
'i' is the definition.
(Roman numeral for one)
'the end of endymion in the middle of poem' is the wordplay.
'the end of' indicates one should take the final letters.
'in the' is an insertion indicator.
'middle of' indicates the central letters.
The centre of 'poem' is 'oe'.
The final letter of 'endymion' is 'n'.
'n' inserted inside 'oe' is 'ONE'.
'put' acts as a link.
(Other definitions for one that I've seen before include "Individual" , "SECOND PART OF 16" , "Half of two" , "Leading cardinal" , "doesn't count for much" .)