Figure shut in with endless pain (8)
I believe the answer is:
pentagon
'figure' is the definition.
(type of geometrical figure)
'shut in with endless pain' is the wordplay.
'shut in' becomes 'pent' (I've seen this in another clue).
'with' is a charade indicator (letters next to each other).
'endless pain' becomes 'agon' (I've seen this in other clues).
'pent'+'agon'='PENTAGON'
(Other definitions for pentagon that I've seen before include "Geometric figure with five sides" , "top brass" , "Five-sided polygon" , "military building" , "Headquarters of USA Department of Defence" .)