Principal in hospital after car crash (4)
I believe the answer is:
arch
'principal' is the definition.
(as in arch-enemy)
'hospital after car crash' is the wordplay.
'hospital' becomes 'h' (abbreviation for hospital on maps).
'after' means one lot of letters go next to another.
'crash' indicates anagramming the letters.
'car' anagrammed gives 'arc'.
'h' put after 'arc' is 'ARCH'.
'in' is the link.
(Other definitions for arch that I've seen before include "(archaic)" , "Prefix meaning chief - maybe an angel" , "Curved shape that spans an opening" , "Clever" , "Instep of foot" .)