Keep retina moving (6)
I believe the answer is:
retain
'keep' is the definition.
(synonyms)
'retina moving' is the wordplay.
'moving' is an anagram indicator (letters are moved around).
'retina' is an anagram of '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" .)