Examine source of contagion in medical facility (4)
I believe the answer is:
scan
'examine' is the definition.
(I know that scan is a kind of examine)
'source of contagion in medical facility' is the wordplay.
'source of' indicates taking the first letters (the start - or source - of the word).
'in' indicates putting letters inside.
'medical facility' becomes 'san' (short for sanatorium).
The initial letter of 'contagion' is 'c'.
'c' put within 'san' is 'SCAN'.
(Other definitions for scan that I've seen before include "Scrutinise; glance over" , "Traverse with a detector" , "convert metres into feet?" , "Survey - examine" , "Study intently" .)