It gives a false impression to one’s pupils (7,8)
I believe the answer is:
optical illusion
'it' is the definition.
The definition suggests a singular noun which matches the answer.
'a false impression to one's pupils' is the wordplay.
'a false impression' becomes 'illusion' (I've seen this before).
'to' says to put letters next to each other (I've seen this in other clues).
'one's pupils' becomes 'optical' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'illusion' put after 'optical' is 'OPTICAL ILLUSION'.
'gives' acts as a link.
(Other definitions for optical illusion that I've seen before include "Mirage" , "Something deceiving the eye" , "outwardly deceptive" , "trick" , "One can't really see it" .)