Jelly, a second choice, ultimately unavailable (5)
I believe the answer is:
aspic
'jelly' is the definition.
(I know that aspic is a type of aspic)
'a second choice ultimately unavailable' is the wordplay.
I cannot really understand how this works, but
'a' is within the answer.
'second' could be 's' (common abbreviation) and 's' is found within the answer.
The remaining letters 'pic' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for aspic that I've seen before include "Preservative jelly" , "Lavender" , "Stock set with food" , "Edible jelly" , "Clear meat or fish jelly" .)