A friend has texted 'you are not professional' (7)
I believe the answer is:
amateur
'not professional' is the definition.
('amateur' is the opposite to 'professional')
'a friend has texted you are' is the wordplay.
'friend' becomes 'mate' (mate is a kind of friend).
'has' means one lot of letters go next to another.
'texted you are' becomes 'ur' (text abbreviation for 'you are').
'a'+'mate'+'ur'='AMATEUR'
(Other definitions for amateur that I've seen before include "Unprofessional" , "lacking professional expertise" , "Inept, non-professional" , "Non-pro" , "I'm no expert!" .)