Boss with itch reveals rash (8)
I believe the answer is:
headlong
'rash' is the definition.
(I've seen this before)
'boss with itch' is the wordplay.
'boss' becomes 'head' (synonyms).
'with' means one lot of letters go next to another.
'itch' becomes 'long'.
'head'+'long'='HEADLONG'
'reveals' acts as a link.
(Other definitions for headlong that I've seen before include "In hasty or plunging manner" , "without thinking?" , "With reckless haste" , "Quickly" , "Precipitate (flight)" .)