Professional impression is ultimately important (7)
I believe the answer is:
dentist
'professional' is the definition.
(dentist is a kind of professional)
'impression is ultimately important' is the wordplay.
'impression' becomes 'dent' (dent is a kind of impression).
'ultimately' indicates one should take the final letters.
The last letter of 'important' is 't'.
'dent'+'is'+'t'='DENTIST'
(Other definitions for dentist that I've seen before include "expert in oral examination" , "Someone who drills for a living?" , "One who treats teeth" , "Stinted (anag.)" , "Practitioner who looks down in the mouth?" .)