Muscle initially bulging in chaps exercising pretty solidly (6)
I believe the answer is:
biceps
'muscle' is the definition.
(biceps is a kind of muscle)
'initially bulging in chaps exercising pretty solidly' is the wordplay.
'initially' suggests taking the first letters.
The first letters of 'bulging in chaps exercising pretty solidly' is 'BICEPS'.
(Other definitions for biceps that I've seen before include "One operating arm" , "Contractor" , "Arm muscles" , "Forearm-flexing muscle" , "I bend the elbow" .)