Treat with acid and the rest before hospital (4)
I believe the answer is:
etch
'treat with acid' is the definition.
'and the rest before hospital' is the wordplay.
'and the rest' becomes 'etc' (et cetera).
'before' is a charade indicator (letters next to each other).
'hospital' becomes 'h' (abbreviation for hospital on maps).
'etc'+'h'='ETCH'
(Other definitions for etch that I've seen before include "What most engravers presumably find easy to do" , "Carve into metal with acid" , "Carve (design) on a surface" , "Wear away by chemical reaction" , "Mark" .)