Colleague in gold rush at first an enthusiast (7)
I believe the answer is:
amateur
'an enthusiast' is the definition.
(I've seen this before)
'colleague in gold rush at first' is the wordplay.
'colleague' becomes 'mate' (I've seen this in another clue).
'in' means one lot of letters goes inside another.
'gold' becomes 'Au' (Au is the chemical symbol for gold).
'at first' indicates taking the first letters.
The first letter of 'rush' is 'r'.
'mate' going into 'au' is 'amateu'.
'amateu'+'r'='AMATEUR'
(Other definitions for amateur that I've seen before include "Lover" , "Unpaid (sportsman}" , "Unpaid player" , "Dilettante: buff" , "Unpaid practitioner" .)