Keen to move small joints (5)
I believe the answer is:
knees
'joints' is the definition.
(knee is a kind of joint)
'keen to move small' is the wordplay.
'to move' indicates anagramming the letters.
'small' becomes 's' (abbreviation - e.g. clothes size).
'keen' anagrammed gives 'knee'.
'knee'+'s'='KNEES'
(Other definitions for knees that I've seen before include "Lap" , "Hinge joints" , "Leg joints" , "Legs' joints" , "Hinged joints" .)