Instruction to reader to preserve drained excess sauce (5)
I believe the answer is:
pesto
'sauce' is the definition.
(I know that pesto is a sauce)
'instruction to reader to preserve drained excess' is the wordplay.
'instruction to reader' becomes 'PTO' (abbreviation for 'please turn over').
'to preserve' indicates putting letters inside.
'drained' says to hollow out the word (remove centre letters).
'excess' with its middle taken out is 'es'.
'pto' placed around 'es' is 'PESTO'.
(Other definitions for pesto that I've seen before include "Sauce of basil, pine nuts, etc" , "Basil-based sauce" , "Pasta sauce mainly of basil and cheese" , "Saucy work" , "Basic sauce" .)