The most common and most likely answer to this crossword clue is the 12 letter word GLASS-BLOWER
I've seen this crossword clue in Independent
G
L
A
S
S
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B
L
O
W
E
R
More clues leading to the result GLASS-BLOWER
Buddy explains!
The straight part
We know that "At work, heavy breather is good girl on phone (5-6)/At work, heavy breather" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case at work, heavy breather is good girl on phone (5-6)/at work, heavy breather = glass-blower.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.