Jack at hostelry serves spirits (4)
I believe the answer is:
jinn
'spirits' is the definition.
(jinni is a kind of spirit)
'jack at hostelry' is the wordplay.
'jack' becomes 'J' (playing card abbreviation).
'at' means one lot of letters go next to another.
'hostelry' becomes 'inn' (synonyms).
'j'+'inn'='JINN'
'serves' is the link.
(Other definitions for jinn that I've seen before include "spirits" , "spirit" .)