The most common and most likely answer to this crossword clue is the 7 letter word RESPIRE
I've seen this crossword clue in The Daily Telegraph
R
E
S
P
I
R
E
More clues leading to the result RESPIRE
Buddy explains!

The straight part
We know that "Breathe" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case breathe = respire.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 RESPIRE mean?
RESPIRE noun- Rest, respite.
- To breathe in and out; to engage in the process of respiration.
- To recover one's breath or breathe easily following stress.
- To (inhale and) exhale; to breathe.