Tolerates professor piercing cells in retina (8)
I believe the answer is:
condones
'tolerates' is the definition.
Although both the answer and definition are verbs in their -s form, I cannot understand how one could define the other.
'professor piercing cells in retina' is the wordplay.
'professor' becomes 'don' (term for a university professor).
'piercing' means one lot of letters goes inside another (some letters pierce a hole in others).
'cells in retina' becomes 'cones' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'don' going within 'cones' is 'CONDONES'.
Can you help me to learn more?
(Another definition for condones that I've seen is " Excuses, overlooks".)