Cafe's penultimate potato dish; one's left in reserve (5)
I believe the answer is:
frost
'reserve' is the definition.
The definition and answer can be both people as well as being singular nouns.
Perhaps there's an association between them I don't understand?
'cafe's penultimate potato dish one's left' is the wordplay.
'cafe's penultimate' becomes 'f' (I can't justify this - if you can you should give a lot more credence to this answer).
'potato dish' becomes 'rosti' (I've seen this before).
'one' becomes 'i' (Roman numeral).
'left' suggests deleting specific letters (some letters have left).
'rosti' with 'i' removed is 'rost'.
'f'+'rost'='FROST'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for frost that I've seen before include "Ice forming in cold weather" , "Frozen dew" , "Coldness of manner" , "Frozen deposit" , "RD Wingfield's detective" .)