A maths problem can cause breathing difficulty (6)
I believe the answer is:
asthma
'breathing difficulty' is the definition.
(I've seen this before)
'a maths problem' is the wordplay.
'problem' indicates an anagram.
'maths' anagrammed gives 'sthma'.
'a'+'sthma'='ASTHMA'
'can cause' is the link.
(Other definitions for asthma that I've seen before include "Physical disorder" , "Shortage of breath" , "Mat has difficult breathing" , "which leaves one breathless" , "Distressing breathing condition" .)