Brandy knocked back by first of patients in pain (5)
I believe the answer is:
cramp
'pain' is the definition.
(I have seen 'Muscular pain' mean 'cramp' so perhaps 'pain' could also mean 'cramp')
'brandy knocked back by first of patients' is the wordplay.
'brandy' becomes 'marc' (marc is a kind of brandy).
'knocked back' says the letters should be written in reverse.
'by' means one lot of letters go next to another.
'first of' says to take the initial letters.
The initial letter of 'patients' is 'p'.
'marc' reversed gives 'cram'.
'cram'+'p'='CRAMP'
'in' is the link.
(Other definitions for cramp that I've seen before include "Confine - muscular spasm" , "restraint" , "writers may be familiar with" , "To restrict; a muscle pain" , "development of vice" .)