Bother! Last of tomato sauce (5)
I believe the answer is:
pesto
'sauce' is the definition.
(I've seen this before)
'bother last of tomato' is the wordplay.
'bother' becomes 'pest'.
'last of' indicates one should take the final letters.
The final letter of 'tomato' is 'o'.
'pest'+'o'='PESTO'
(Other definitions for pesto that I've seen before include "Sauce of basil, pine nuts, etc" , "Sauce made of basil and cheese" , "A sauce served with pasta" , "Basic sauce" , "Pasta sauce mainly of basil and cheese" .)