Quick to show a professional's not upset (6)
I believe the answer is:
pronto
'quick' is the definition.
(I've seen this in another clue)
'a professional's not upset' is the wordplay.
'a professional' becomes 'pro' (abbreviation).
'upset' indicates an anagram.
'not' is an anagram of 'nto'.
'pro'+'nto'='PRONTO'
'to show' is the link.
(Other definitions for pronto that I've seen before include "Promptly, without delay" , "Italian telephonic greeting" , "Right away, quickly" , "Quickly (sl.)" , "Immediately, amigo" .)