Muscle aches regularly introduced by stumbles (7)
I believe the answer is:
triceps
'muscle' is the definition.
(triceps is a kind of muscle)
'aches regularly introduced by stumbles' is the wordplay.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'introduced by' means one lot of letters goes inside another (I've seen this in other clues).
'stumbles' becomes 'trips' ('trip' can be a synonym of 'stumble').
The alternating letters of 'aches' are 'ce'.
'ce' placed within 'trips' is 'TRICEPS'.
(Other definitions for triceps that I've seen before include "one can't bend the elbow!" , "Muscle at the back of the upper arm" , "Green citrus fruit" , "Upper-arm muscle" , "army feature?" .)