Fast, not eating rare sauce (5)
I believe the answer is:
pesto
'sauce' is the definition.
(I know that pesto is a sauce)
'fast not eating rare' is the wordplay.
I cannot really see how this works, but
'not' could be 'o' (I've seen this in other clues) and 'o' is found in the answer.
The remaining letters 'pest' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for pesto that I've seen before include "Green sauce" , "Basil and pine nut sauce" , "Saucy work" , "Pasta sauce variety" , "Sauce of basil, pine nuts, etc" .)