It shows the pupils are in need of a rest (9)
I believe the answer is:
eyestrain
'it shows the pupils' is the definition.
The definition suggests a singular noun which matches the answer.
'in need of a rest' is the wordplay.
I cannot quite see how this works, but
'in' is found in the answer.
'a' is found in the answer.
an anagram of 'rest' is 'estr' which is present in the answer.
This may be the basis of the clue (or it may be nonsense).
'are' is the link.
Can you help me to learn more?
(Other definitions for eyestrain that I've seen before include "You're looking too hard for this" , "Discomfort of sightseers" , "Vision problem" , "Tiredness when reading" , "complaint" .)