Keep retina moving (6)
I believe the answer is:
retain
'keep' is the definition.
(I know that keep can be written as retain)
'retina moving' is the wordplay.
'moving' indicates anagramming the letters (letters are moved around).
'retina' anagrammed gives 'RETAIN'.
(Other definitions for retain that I've seen before include "Hold onto" , "'Hold, keep back (6)'" , "Keep in Tirane" , "Not relinquish" , "Hold on to or employ counsel" .)