Stumbles around beach regularly twisting muscle (7)
I believe the answer is:
triceps
'muscle' is the definition.
(triceps is a kind of muscle)
'stumbles around beach regularly twisting' is the wordplay.
'stumbles' becomes 'trips' ('trip' can be a synonym of 'stumble'**).
'around' means one lot of letters goes inside another.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'twisting' indicates an anagram.
The alternating letters of 'beach' are 'ec'.
'ec' anagrammed gives 'ce'.
'trips' placed around 'ce' is 'TRICEPS'.
(Other definitions for triceps that I've seen before include "Green citrus fruit" , "Source of uplifting strength" , "Extensor muscle" , "Muscle that extends the forearm" , "Muscle at the back of the upper arm" .)