Underwear that is contributing to heavy breathing (7)
I believe the answer is:
panties
'heavy breathing' is the definition.
The answer and definition are not the same part of speech.
'underwear that is contributing to' is the wordplay.
'underwear' becomes 'pants' (underpants).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'contributing to' indicates putting letters inside (inserted letters make a new contribution).
'pants' placed around 'ie' is 'PANTIES'.
Can you help me to learn more?
(Another definition for panties that I've seen is " Clothes".)