A first-class average reflects weakness (7)
I believe the answer is:
anaemia
'weakness' is the definition.
(condition causing weakness)
'a first-class average reflects' is the wordplay.
'first class' becomes 'ai' (A1 - top quality).
'average' becomes 'mean' (type of average).
'reflects' says the letters should be written backwards.
'ai'+'mean'='aimean'
'aimean' back-to-front is 'naemia'.
'a'+'naemia'='ANAEMIA'
(Other definitions for anaemia that I've seen before include "Red blood cell deficiency" , "Disorder of deficiency in red blood cells" , "Blood deficiency condition" , "Llama relative, its wool" , "Bloodlessness" .)