The most common and most likely answer to this crossword clue is the 8 letter word POTBOUND
I've seen this crossword clue in The Guardian
P
O
T
B
O
U
N
D
More clues leading to the result POTBOUND
Buddy explains!
The straight part
We know that "too big for its container" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case too big for its container = potbound.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 POTBOUND mean?
POTBOUND adjective- (of a potted plant) Having outgrown the size of the pot, such that its roots have no more room to grow.