To boot a number outside hospital (4)
I believe the answer is:
then
'to boot' is the definition.
(thesaurus)
'a number outside hospital' is the wordplay.
'a number' becomes 'ten' (ten is a kind of number).
'outside' means one lot of letters goes inside another.
'hospital' becomes 'h'.
'ten' placed around 'h' is 'THEN'.
(Other definitions for then that I've seen before include "Consequently" , "At that time; next" , "Subsequently - previously" , "Once upon a time" , "Afterwards - as a result" .)