All our letters after B: A goes, for a start (7)
I believe the answer is:
postbag
'a start' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'all our letters after b a goes' is the wordplay.
I cannot really understand how this works, but
'letters' could be 'post' (letters in the mail or post) and 'post' is found within the answer.
'b' is found in the answer.
'a' is found in the answer.
A single letter 'g' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
'for' is the link.
(Other definitions for postbag that I've seen before include "carrier for Royal Mail" , "one's letters" , "Letters received" , "What one receives collectively" , "All delivered" .)