Run into section of the fortification (7)
I believe the answer is:
rampart
'the fortification' is the definition.
(rampart is a kind of fortification)
'run into section' is the wordplay.
'run into' becomes 'ram' (I've seen this in another clue).
'section' becomes 'part' (synonyms).
'ram'+'part'='RAMPART'
'of' acts as a link.
(Other definitions for rampart that I've seen before include "Pain-relieving drug" , "barricade" , "defencework" , "Fortified wall" , "Embankment built for defensive purposes" .)