Bug found on old sauce (5)
I believe the answer is:
pesto
'sauce' is the definition.
(pesto is a type of sauce)
'bug found on old' is the wordplay.
'bug' becomes 'pest' (pest can mean an insect).
'found on' means one lot of letters go next to another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'pest'+'o'='PESTO'
(Other definitions for pesto that I've seen before include "Sauce of basil, pine nuts, etc" , "Ligurian food" , "Sauce of basil, pine nuts and garlic" , "Pasta sauce based on basil" , "Pasta sauce variety" .)