Complaint about Ma's hat (6)
I believe the answer is:
asthma
'complaint' is the definition.
(asthma is a medical complaint)
'about ma's hat' is the wordplay.
'about' is an anagram indicator.
'mas'+'hat'='mashat'
'mashat' anagrammed gives 'ASTHMA'.
(Other definitions for asthma that I've seen before include "Distressing ailment affecting breathing" , "Breathing problem" , "Respiratory problem" , "Respiratory disease" , "Lung condition" .)