Joint cry of alarm upset nervous beginner inside (4)
I believe the answer is:
knee
'joint' is the definition.
(knee joint is a kind of joint)
'cry of alarm upset nervous beginner inside' is the wordplay.
'cry of alarm' becomes 'eek' (I've seen this before).
'upset' says the letters should be written in reverse.
'beginner' indicates taking the first letters.
'inside' indicates putting letters inside.
The initial letter of 'nervous' is 'n'.
'eek' back-to-front is 'kee'.
'kee' going around 'n' is 'KNEE'.
(Other definitions for knee that I've seen before include "proposal might be made on one" , "Hinged joint" , "Hinge between the thigh and the shin" , "for going on begging" , "Wounded in Indian Wars action" .)