Hospital margin is a fence? (5)
I believe the answer is:
hedge
'a fence?' is the definition.
(hedge is a kind of fence)
'hospital margin' is the wordplay.
'hospital' becomes 'H' (abbreviation for hospital).
'margin' becomes 'edge' (synonyms).
'h'+'edge'='HEDGE'
'is' acts as a link.
(Other definitions for hedge that I've seen before include "Be cautious" , "Fence formed by bushes" , "Financial contract" , "cover?" , "avoid making commitments" .)