I retain movement in the stillness (7)
I believe the answer is:
inertia
'stillness' is the definition.
(I've seen this before)
'i retain movement in' is the wordplay.
'movement' indicates an anagram (letters move around).
'in' means one lot of letters goes inside another.
'retain' anagrammed gives 'inerta'.
'i' put into 'inerta' is 'INERTIA'.
'the' is the link.
(Other definitions for inertia that I've seen before include "Way of selling" , "Tendency not to act" , "Stagnating state" , "showing reluctance to move" , "I retain, being without energy" .)