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' suggests the final letters.
'in the' indicates putting letters inside.
'middle of' indicates the central letters.
The central letters of 'poem' are 'oe'.
The last letter of 'endymion' is 'n'.
'n' placed 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" .)