Genteel fool's badly neglected appearance (3,7,2)
I believe the answer is:
let oneself go
'neglected appearance' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.
'genteel fool's badly' is the wordplay.
'badly' indicates anagramming the letters (letters in the wrong order).
'genteel'+'fools'='genteelfools'
'genteelfools' with letters rearranged gives 'LET ONESELF GO'.
Can you help me to learn more?
(Other definitions for let oneself go that I've seen before include "Neglect one's appearance" , "become abandoned" , "Act in an uninhibited manner - lose interest in one's appearance" , "Genteel fools (anag) - act without restraint" .)