A tricky maths problem (6)
I believe the answer is:
asthma
'problem' is the definition.
(I have seen 'Respiratory problem ' mean 'asthma' so perhaps 'problem' could also mean 'asthma')
'a tricky maths' is the wordplay.
'tricky' indicates an anagram.
'maths' anagrammed gives 'sthma'.
'a'+'sthma'='ASTHMA'
(Other definitions for asthma that I've seen before include "Breathing complaint" , "Mat has difficult breathing" , "Physical disorder" , "Respiratory disorder with wheezing" , "Respiratory problem" .)