Article on complex maths leads to complaint (6)
I believe the answer is:
asthma
'complaint' is the definition.
(asthma is a kind of disease)
'article on complex maths' is the wordplay.
'article' becomes 'a' (indefinite article**).
'on' indicates putting letters inside.
'complex' is an anagram indicator.
'maths' is an anagram of 'sthma'.
'a' placed into 'sthma' is 'ASTHMA'.
'leads to' is the link.
(Other definitions for asthma that I've seen before include "Strange that Mat has difficulty breathing" , "inspiring trouble" , "Mat has difficult breathing" , "Bronchial allergy" , "Mat has a breathing disorder" .)