Doctor taking in most of sick and faint (4)
I believe the answer is:
mild
'faint' is the definition.
(thesaurus)
'doctor taking in most of sick' is the wordplay.
'doctor' becomes 'md' ('MD' can be a synonym of 'doctor'**).
'taking in' indicates putting letters inside.
'most of' means to remove the last letter.
'sick' becomes 'ill' ('ill' can be a synonym of 'sick').
'ill' with its final letter taken off is 'il'.
'md' going around 'il' is 'MILD'.
'and' acts as a link.
(Other definitions for mild that I've seen before include "Moderately warm - type of beer" , "Clement -- type of beer" , "Dark beer, not strong in flavour" , "Balmy or moderate" , "Beer darker than bitter - not bitter" .)