Muscle boosting ingredient can enhance players' skills initially (6)
I believe the answer is:
biceps
'muscle' is the definition.
(biceps is a kind of muscle)
'boosting ingredient can enhance players' skills initially' is the wordplay.
'initially' indicates taking the first letters.
The first letters of 'boosting ingredient can enhance players skills' is 'BICEPS'.
(Other definitions for biceps that I've seen before include "Arm muscles" , "Arm workers" , "One operating arm" , "Elbow-bending muscle" , "Forearm-flexing muscle" .)