Periodically uncouth? Anything but (3)
I believe the answer is:
not
'anything but' is the definition.
(I've seen this in other clues)
'periodically uncouth?' is the wordplay.
'periodically' indicates alternate letters (letters must be selected 'every so often').
The alternating letters of 'uncouth' are 'NOT'.
(Other definitions for not that I've seen before include "logically, operation needed" , "Ton (anag.)" , "Negation of what follows" , "Negatory word" , "Type of logic gate" .)