Source of infection in obvious singular cases (8)
I believe the answer is:
patients
'cases' is the definition.
('patient' can be a synonym of 'case')
'source of infection in obvious singular' is the wordplay.
'source of' indicates taking the first letters.
'in' is an insertion indicator.
'obvious' becomes 'patent' ('patent' can be similar in meaning to 'obvious'**).
'singular' becomes 's' (abbreviation sometimes used in dictionaries).
The initial letter of 'infection' is 'i'.
'i' going within 'patent' is 'patient'.
'patient'+'s'='PATIENTS'
(Other definitions for patients that I've seen before include "I sent Pat into the hospital inmates" , "Doctors' customers" , "People being treated by doctors, may be in hospital" , "They're treated" , "People under the doctor" .)