Nonprofessional friend in gold on river (7)
I believe the answer is:
amateur
'nonprofessional' is the definition.
(similar in meaning)
'friend in gold on river' is the wordplay.
'friend' becomes 'mate' (mate is a kind of friend).
'in' is an insertion indicator.
'gold' becomes 'Au' (Au is the chemical symbol for gold).
'on' says to put letters next to each other.
'river' becomes 'r' (abbreviation).
'mate' placed inside 'au' is 'amateu'.
'amateu'+'r'='AMATEUR'
(Other definitions for amateur that I've seen before include "Layman" , "One is not professional" , "Engaged in as a pastime" , "Ham" , "Lover" .)