Collection of letters in which X frequently appears? (6,6)
I believe the answer is:
postal ballot
'appears?' is the definition.
I know nothing about this answer so I cannot judge whether this works.
'collection of letters in which x frequently' is the wordplay.
I cannot really understand how this works, but
'collection' could be 'ball' (ball is a kind of collection) and 'ball' is found in the answer.
'of' could be 'o' and 'o' is found within the answer.
'frequently' could be 'alot' (I've seen this in another clue) and 'alot' is found in the leftover letters.
The remaining letters 'pst' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
(Other definitions for postal ballot that I've seen before include "Voting method" , "Form of election" .)