An old copper, thing chucked a great distance (4)
I believe the answer is:
'a great distance' is the definition.
'an old copper thing chucked' is the wordplay.
'an' becomes 'a'.
'old copper' becomes 'farthing' (former UK coin).
'chucked' is a deletion indicator.
'farthing' with 'thing' removed is 'far'.
(Other definitions for afar that I've seen before include "long way back" , "(From/at) a great distance" , "Not in this neighbourhood" , "Way-out" , "A long way away" .)