Setter's favourite: "Upstanding old soldier's complaint" (8)
I believe the answer is:
impetigo
'complaint' is the definition.
(impetigo is a kind of disease)
'setter's favourite upstanding old soldier's' is the wordplay.
'setter's' becomes 'im' (I've seen this in other clues**).
'favourite' becomes 'pet' ('pet' can be a synonym of 'favourite').
'upstanding' says the letters should be written in reverse.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'soldier' becomes 'gi' (government issue or general issue).
'o'+'gi'='ogi'
'ogi' reversed gives 'igo'.
'im'+'pet'+'igo'='IMPETIGO'
(Other definitions for impetigo that I've seen before include "unsightly condition" , "Skin condition" , "complaint" , "Contagious skin infection common in children" , "sickness" .)