Fonda's forte (8)
I believe the answer is:
aerobics
'fonda's forte' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for aerobics that I've seen before include "Exercises designed to increase oxygen intake" , "Form of exercise" , "Exercises designed to strengthen heart and lungs" , "Strenuous exercises" , "Sort of exercises that make Boris ache badly after an hour's gone" .)