An exercising weight (8)
I believe the answer is:
dumbbell
'an exercising weight' is the definition.
(dumbbell is a kind of exercising weight)
This is the entire clue.
(Other definitions for dumbbell that I've seen before include "Exercising weight" , "Weight-training device" , "Weightlifter's item" , "Short weighted bar for exercises - sounds stupid" , "Weight-training equipment" .)