Keep retina moving (6)
I believe the answer is:
retain
'keep' is the definition.
(retaining is a kind of keeping)
'retina moving' is the wordplay.
'moving' is an anagram indicator (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" .)