Inexpert fellow invested in gold? Heading for ruin (7)
I believe the answer is:
amateur
'inexpert' is the definition.
(I've seen this before)
'fellow invested in gold? heading for ruin' is the wordplay.
'fellow' becomes 'mate' (I've seen this before).
'invested in' means one lot of letters goes inside another.
'gold?' becomes 'Au' (Au is the chemical symbol for gold).
'heading for' indicates taking the first letters.
The first letter of 'ruin' is 'r'.
'mate' put within 'au' is 'amateu'.
'amateu'+'r'='AMATEUR'
(Other definitions for amateur that I've seen before include "Lay person" , "Of sportsman, not professional" , "unskilful" , "Unpaid performer" , "Lover" .)