Exhaustion experienced when observing fine English school dons (9)
I believe the answer is:
eyestrain
'exhaustion experienced when observing' is the definition.
(exhaustion from using one's eyes or observing)
'fine english school dons' is the wordplay.
'fine' becomes 'yes' ('fine' is a way of saying 'yes').
'english' becomes 'e' (abbreviation).
'school' becomes 'train' (training is a kind of schooling).
'dons' is an insertion indicator (don means to wear or put on).
'e'+'train'='etrain'
'yes' inserted into 'etrain' is 'EYESTRAIN'.
(Other definitions for eyestrain that I've seen before include "Discomfort of sightseers" , "pupils are involved in such trouble" , "Reader's problem" , "tiredness of pupils" , "puts pressure on pupils" .)