Troublesome maths — a problem that leaves one breathless (6)
I believe the answer is:
asthma
'problem that leaves one breathless' is the definition.
'asthma' can be an answer for 'problem' (I have seen 'Respiratory problem ' mean 'asthma' so perhaps 'problem' could also mean 'asthma'). I'm not sure about the remainder of the definition.
'troublesome maths a' is the wordplay.
'troublesome' indicates anagramming the letters.
'maths' anagrammed gives 'asthm'.
'asthm'+'a'='ASTHMA'
Can you help me to learn more?
(Other definitions for asthma that I've seen before include "Breathing illness" , "Strange that Mat has difficulty breathing" , "which leaves one breathless" , "Respiratory disease" , "Cause of breathing difficulty" .)