Forces surrounding some fortifications (8)
I believe the answer is:
ramparts
'fortifications' is the definition.
(rampart is a kind of fortification)
'forces surrounding some' is the wordplay.
'forces' becomes 'rams' ('ram' can be a synonym of 'force').
'surrounding' indicates putting letters inside.
'some' becomes 'part' (only part of something).
'rams' placed around 'part' is 'RAMPARTS'.
(Other definitions for ramparts that I've seen before include "Defensive fortifications" , "Earth banks as fortifications topped with parapets" , "used defensively" , "Fortified earth banks topped with parapets" , "Defensive walls with parapets" .)