Wrong to besiege Troy without end (8)
I believe the answer is:
immortal
'without end' is the definition.
(never dying or ending)
'wrong to besiege troy' is the wordplay.
'wrong' becomes 'immoral' (similar in meaning).
'to besiege' means one lot of letters goes inside another.
'troy' becomes 't'.
'immoral' going around 't' is 'IMMORTAL'.
(Other definitions for immortal that I've seen before include "Enduring" , "never to be forgotten" , "Undying" , "Great person" , "Everlasting" .)