Leaves prisoner clutching one, besieged by enemy (7)
I believe the answer is:
foliage
'leaves' is the definition.
(I've seen this before)
'prisoner clutching one besieged by enemy' is the wordplay.
'prisoner' becomes 'lag' (I've seen this before).
'clutching' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'besieged by' means one lot of letters goes inside another (I've seen this in other clues).
'enemy' becomes 'foe' (I've seen this before).
'lag' going around 'i' is 'liag'.
'liag' placed into 'foe' is 'FOLIAGE'.
(Other definitions for foliage that I've seen before include "Plant leaves collectively" , "Green leaves" , "Mass of leaves" , "Greenery, leaves of trees or plants" , "(Decorative) leaves" .)