The most common and most likely answer to this crossword clue is the 7 letter word HOARDER
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More clues leading to the result HOARDER
- More difficult saving nothing — for this person?
- Treasure Her Majesty — she’s a keeper
- I keep everything: retaining nothing is more difficult
- Not so easy housing old compulsive collector
- For person with piles it's more arduous eating egg
- Squirrel of greyish-white turning red
- Collector sporting more impressive erection
- Frost appearing with communist revolutionary, one giving nothing away
- Keeper who saves extravagantly
- Ring more difficult to keep for collector
Buddy explains!

The straight part
We know that "Pack rat" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case pack rat = hoarder.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.
What does HOARDER mean?
HOARDER noun- One who hoards; one who accumulates, collects, and stores, especially one who does so to excess.