Complaint of students backing coup (9)
I believe the answer is:
sunstroke
'complaint' is the definition.
(sunstroke is a kind of health problem)
'students backing coup' is the wordplay.
'students' becomes 'nus' (I've seen this in other clues).
'backing' says the letters should be written in reverse.
'coup' becomes 'stroke' (I've seen this before).
'nus' back-to-front is 'sun'.
'sun'+'stroke'='SUNSTROKE'
'of' is the link.
(Other definitions for sunstroke that I've seen before include "Problem caused by too much solar exposure" , "Malady of UV exposure" , "Illness incurred in trying for tan?" , "illness out of the blue" , "We may avoid it under awning" .)