Muscle problem hit hard when standing up (4)
I believe the answer is:
knot
'muscle problem' is the definition.
(I've seen this in another clue)
'hit hard when standing up' is the wordplay.
'hit hard' becomes 'tonk' (to tonk is to hit forcefully).
'when standing up' shows that the letters should be reversed in order (in a down clue, letters go up).
'tonk' backwards is 'KNOT'.
(Other definitions for knot that I've seen before include "Entangle" , "Tie; a cluster" , "one may find in wood" , "Fastening formed by tying string etc" , "Bird; hard part in wood" .)