Start, not finish, at site of siege (5)
I believe the answer is:
intro
'start' is the definition.
(I've seen this before)
'not finish at site of siege' is the wordplay.
I cannot really see how this works, but
'not' could be 'nt' ('n't' can be a synonym of 'not') and 'nt' is found within the answer.
'of' could be 'o' and 'o' is found within the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for intro that I've seen before include "First bars of a musical composition" , "starter" , "Beginning (informal)" , "Opening bars of a song" , "Instrumental part at start of song" .)