The most common and most likely answer to this crossword clue is the 8 letter word TESTICLE
I've seen this crossword clue in Independent
T
E
S
T
I
C
L
E
Buddy explains!

The straight part
We know that "seed store" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case seed store = testicle.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 TESTICLE mean?
TESTICLE noun- The male sex and endocrine gland, found in some types of animals, that produces sperm and male sex hormones, including the steroid testosterone.