Complaint when math goes wrong (6)
I believe the answer is:
asthma
'complaint' is the definition.
(asthma is a medical complaint)
'when math goes wrong' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'goes wrong' indicates an anagram.
'math' anagrammed gives 'thma'.
'as'+'thma'='ASTHMA'
(Other definitions for asthma that I've seen before include "Respiratory illness" , "Respiratory disorder with wheezing" , "Mat has a respiratory disorder" , "Respiratory problem" , "Breathing problem" .)