The most common and most likely answer to this crossword clue is the 10 letter word OVERWEIGHT
I've seen this crossword clue in The Daily Telegraph
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More clues leading to the result OVERWEIGHT
Buddy explains!

The straight part
We know that "too heavy" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case too heavy = overweight.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 OVERWEIGHT mean?
OVERWEIGHT noun- An excess of weight.
- An overweight person.
- (investment) A security or class of securities in which one has a heavy concentration.
- To weigh down: to put too heavy a burden on.
- To place excessive weight or emphasis on; to overestimate the importance of.
- (of a person) Heavier than what is generally considered healthy for a given body type and height.
- (of a vehicle) Weighing more than what is allowed for safety or legal commerce.
- (investment, followed by a noun or prepositional phrase indicating a security or type of security) Having a portfolio relatively heavily invested in.