Muscle having instant power? Singular (7)
I believe the answer is:
triceps
'muscle' is the definition.
(triceps is a kind of muscle)
'instant power? singular' is the wordplay.
'instant' becomes 'trice' (synonyms).
'power?' becomes 'p' (symbol in physics).
'singular' becomes 's' (abbreviation sometimes used in dictionaries).
'trice'+'p'+'s'='TRICEPS'
'having' is the link.
(Other definitions for triceps that I've seen before include "Source of uplifting strength" , "army feature?" , "Muscle at the back of the upper arm" , "Upper-arm muscle" , "one can't bend the elbow!" .)