Old coin; about a pound (4)
I believe the answer is:
real
'old coin' is the definition.
'real' can be an answer for 'coin' (real is a kind of coin). I am unsure of the 'old' bit.
'about a pound' is the wordplay.
'about' becomes 'rea' (I've seen this in other clues).
'a pound' becomes 'l' (abbreviation e.g. Lsd - pounds, shillings, pence).
'rea'+'l'='REAL'
Can you help me to learn more?
(Other definitions for real that I've seen before include "Existing, not imaginery" , "In America, this estate comprises land and buildings" , "Genuine or existing" , "'Genuine, actual (4)'" , "Pukka" .)