Charlie in sanatorium for a test (4)
I believe the answer is:
scan
'a test' is the definition.
(a medical scan is a type of test)
'charlie in sanatorium' is the wordplay.
'charlie' becomes 'c' (phonetic alphabet: alpha, bravo, charlie etc.).
'in' means one lot of letters goes inside another.
'sanatorium' becomes 'san'.
'c' going into 'san' is 'SCAN'.
'for' acts as a link.
(Other definitions for scan that I've seen before include "Perusal" , "Examine, view" , "Look over cursorily" , "Read electronically" , "Survey - check-up" .)