Tossing caber, so I exercise (8)
I believe the answer is:
aerobics
'exercise' is the definition.
(aerobics is a kind of exercise)
'tossing caber so i' is the wordplay.
'tossing' is an anagram indicator.
'caber'+'so'+'i'='cabersoi'
'cabersoi' is an anagram of 'AEROBICS'.
(Other definitions for aerobics that I've seen before include "Keep-fit exercises" , "System of exercises" , "Form of exercises" , "Physical exercises" , "Sort of exercises that make Boris ache badly after an hour's gone" .)