Rash — with smell — concealing a strange mark (5-6)
I believe the answer is:
harum-scarum
'rash' is the definition.
(I've seen this before)
'smell concealing a strange mark' is the wordplay.
'smell' becomes 'hum' (something with a bad smell is 'humming').
'concealing' is an insertion indicator.
'strange' becomes 'rum' (similar in meaning).
'mark' becomes 'scar' (mark left by an injury).
'a'+'rum'+'scar'='arumscar'
'hum' enclosing 'arumscar' is 'HARUM-SCARUM'.
'with' acts as a link.
(Other definitions for harum-scarum that I've seen before include "Cheerfully irresponsible" , "Rash" , "Reckless, impetuous" , "Wild and reckless" , "Wild and thoughtless" .)