Stumbled, initially terribly muscular (7)
I believe the answer is:
tripped
'stumbled' is the definition.
('trip' can be a synonym of 'stumble')
'initially terribly muscular' is the wordplay.
'initially' says to take the initial letters.
'muscular' becomes 'ripped' (I've seen this in another clue).
The first letter of 'terribly' is 't'.
't'+'ripped'='TRIPPED'
(Other definitions for tripped that I've seen before include "Came a cropper" , "Missed a step, stumbled" , "Took acid" .)