Eagerly buy something worthless on back of tin (4,2)
I believe the answer is:
snap up
'eagerly buy' is the definition.
'snap up' can be an answer for 'buy' (to snap something up is to buy it quickly). I am not sure about the 'eagerly' bit.
'something worthless on back of tin' is the wordplay.
I cannot really understand how this works, but
'tin' could be 'sn' (Sn is the chemical symbol for tin) and 'sn' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for snap up that I've seen before include "Unhesitatingly take" , "Buy (a rarity) quickly" , "Purchase eagerly and quickly" , "Snatch" , "Quickly accept" .)