Spare money to keep hospital (4)
I believe the answer is:
thin
'spare' is the definition.
('thin' can be similar in meaning to 'spare')
'money to keep hospital' is the wordplay.
'money' becomes 'tin' (I've seen this before).
'to keep' means one lot of letters goes inside another.
'hospital' becomes 'h'.
'tin' going around 'h' is 'THIN'.
(Other definitions for thin that I've seen before include "Slender, narrow" , "(Of smile) weak, forced" , "Measure" , "Skinny" , "Spare" .)