Keep the retina active (6)
I believe the answer is:
retain
'keep' is the definition.
(I know that keep can be written as retain)
'retina active' is the wordplay.
'active' indicates anagramming the letters (the letters are active/moving).
'retina' is an anagram of 'RETAIN'.
'the' acts as a link.
(Other definitions for retain that I've seen before include "Hire - keep" , "Continue to hold on to" , "Keep; remember" , "Hold in position" , "Not relinquish" .)