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' says to take the final letters.
'in the' means one lot of letters goes inside another.
'middle of' means to look at the middle letters.
The middle letters of 'poem' are 'oe'.
The final letter of 'endymion' is 'n'.
'n' placed within '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" .)