Bewail mental defect (6)
I believe the answer is:
lament
'bewail' is the definition.
(I know that bewail can be written as lament)
'mental defect' is the wordplay.
'defect' indicates anagramming the letters (a defective version of the letters).
'mental' anagrammed gives 'LAMENT'.
(Other definitions for lament that I've seen before include "Elegy" , "Mental (anag)" , "complaint" , "Dart-throwing line" , "Expression of sorrow" .)