Sell, in small quantities, rarest metals whilst in centre (6)
I believe the answer is:
retail
'sell in small quantities' is the definition.
(I've seen this before)
'rarest metals whilst in centre' is the wordplay.
'rarest metals' becomes 'reta' (I can't explain this - if you can you should believe this answer much more).
'in centre' means to look at the middle letters.
The middle of 'whilst' is 'il'.
'reta'+'il'='RETAIL'
Can you help me to learn more?
(Other definitions for retail that I've seen before include "The selling of goods to consumers" , "Sell direct to the customer" , "Sell; pass on (e.g. gossip)" , "Flog" , "This trade sells to the general public" .)