Unprofessional friend fills gold with regular lead (7)
I believe the answer is:
amateur
'unprofessional' is the definition.
(synonyms)
'friend fills gold with regular lead' is the wordplay.
'friend' becomes 'mate' (mate is a kind of friend).
'fills' means one lot of letters goes inside another.
'gold' becomes 'Au' (Au is the chemical symbol for gold).
'with' says to put letters next to each other.
'lead' suggests taking the first letters.
The first letter of 'regular' is 'r'.
'mate' going into 'au' is 'amateu'.
'amateu'+'r'='AMATEUR'
(Other definitions for amateur that I've seen before include "incompetent" , "Unprofessional" , "Receiving no money" , "Unpaid (sports)" , "hobbyist" .)