Reached last of peanut butter sandwiches (3,2)
I believe the answer is:
got at
'reached' is the definition.
(getting at is a kind of reaching)
'last of peanut butter sandwiches' is the wordplay.
'last of' says to take the final letters.
'butter' becomes 'goat' (a goat is a butt-er).
'sandwiches' is an insertion indicator (to sandwich is to block something in).
The last letter of 'peanut' is 't'.
't' put within 'goat' is 'GOT AT'.
(Other definitions for got at that I've seen before include "Unfairly influenced" , "Nobbled" , "Criticised" , "Attacked verbally" .)