Ah! Pistol goes off in infirmary (8)
I believe the answer is:
hospital
'infirmary' is the definition.
('hospital' can be a synonym of 'infirmary')
'ah pistol goes off' is the wordplay.
'goes off' indicates an anagram.
'ah'+'pistol'='ahpistol'
'ahpistol' anagrammed gives 'HOSPITAL'.
'in' is the link.
(Other definitions for hospital that I've seen before include "Infirmary" , "Institute for care and treatment of patients" , "Medical centre" , "Treatment and care centre" , "Possibly Guy's" .)