A tricky maths complaint (6)
I believe the answer is:
asthma
'complaint' is the definition.
(asthma is a kind of disease)
'a tricky maths' is the wordplay.
'tricky' indicates an anagram.
'maths' with letters rearranged gives 'sthma'.
'a'+'sthma'='ASTHMA'
(Other definitions for asthma that I've seen before include "which leaves one breathless" , "Chronic breathing condition" , "Strange that Mat has difficulty breathing" , "Bronchial allergy" , "Respiratory condition" .)