The most common and most likely answer to this crossword clue is the 7 plus 6 letter word POSTAGE STAMPS
I've seen this crossword clue in The Times
P
O
S
T
A
G
E
S
T
A
M
P
S
More clues leading to the result POSTAGE STAMPS
Buddy explains!
The straight part
We know that "Frank often replaces these collector's items" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case frank often replaces these collector's items = postage stamps.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.