Thrust sections into the fortifications (8)
I believe the answer is:
ramparts
'the fortifications' is the definition.
(rampart is a kind of fortification)
'thrust sections' is the wordplay.
'thrust' becomes 'ram' (ramming is a kind of thrusting).
'sections' becomes 'parts' ('part' can be a synonym of 'section').
'ram'+'parts'='RAMPARTS'
'into' is the link.
(Other definitions for ramparts that I've seen before include "Earth banks as fortifications topped with parapets" , "Fortified earth banks topped with parapets" , "defence" , "Sheep bits for fortifications" , "Defensive fortifications" .)