Small coin is old — chuck back (4)
I believe the answer is:
obol
'small coin' is the definition.
'Obol' can be an answer for 'coin' (I've seen this before). I'm not sure about the 'small' bit.
'old chuck back' is the wordplay.
'old' becomes 'o' (abbreviation).
'chuck' becomes 'lob' (I've seen this before).
'back' says the letters should be written backwards.
'lob' written backwards gives 'bol'.
'o'+'bol'='OBOL'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for obol that I've seen before include "Small currency" , "bit of Ancient Greece" , "Jargon, slang" , "Old Greek coin" , "Ancient Greek coin, six to the drachma" .)