Criticise a mother's hat (6)
I believe the answer is:
panama
'hat' is the definition.
(panama is a kind of hat)
'criticise a mother's' is the wordplay.
'criticise' becomes 'pan' (informal term meaning to criticise heavily).
'mother' becomes 'ma' (Ma is another word for mother or mum).
'pan'+'a'+'ma'='PANAMA'
(Other definitions for panama that I've seen before include "What may be seen overhead" , "Land - hat - cigar" , "Central American country" , "Canal state; hat" , "'Country, canal and hat (6)'" .)