Most of retina dreadfully unreactive (5)
I believe the answer is:
inert
'unreactive' is the definition.
(similar in meaning)
'most of retina dreadfully' is the wordplay.
'most of' means to remove the last letter.
'dreadfully' indicates an anagram (a dreadful spelling).
'retina' with its last letter taken off is 'retin'.
'retin' with letters rearranged gives 'INERT'.
(Other definitions for inert that I've seen before include "Sluggish, inactive" , "Slow and apathetic" , "like nitrogen" , "Disinclined to act" , "Lifeless, sluggish" .)