Unit of heat, one of many counted by weight-watcher (7)
I believe the answer is:
calorie
'unit of heat' is the definition.
(I've seen this in another clue)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for calorie that I've seen before include "Unit of heat energy" , "Unit of heat (concerned with diet)" , "Food energy unit" , "Unit of energy in foods" , "Oil race (anag)" .)