Huge group of soldiers seizing part of hospital — big shock may be felt there (9)
I believe the answer is:
epicentre
'big shock may be felt there' is the definition.
(as in the epicentre of an earthquake)
'huge group of soldiers seizing part of hospital' is the wordplay.
'huge' becomes 'epic' (I've seen this before).
'group of soldiers' becomes 'RE' (abbreviation for Royal Engineers).
'seizing' means one lot of letters goes inside another.
'part of hospital' becomes 'ent' (abbreviation for Ear, Nose and Throat department).
'epic'+'re'='epicre'
'epicre' enclosing 'ent' is 'EPICENTRE'.
(Other definitions for epicentre that I've seen before include "Point at which earthquake reaches earth's surface" , "in the middle" , "Focus of activity" , "most shocking point" , "Area most affected" .)