Namely missing old idiot (4)
I believe the answer is:
twit
'idiot' is the definition.
(I've seen this in other clues)
'namely missing old' is the wordplay.
'namely' becomes 'towit' (I've seen this before).
'missing' indicates named letters should be taken away.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'towit' with 'o' taken away is 'TWIT'.
(Other definitions for twit that I've seen before include "Mild word for a fool" , "Silly person (colloq.)" , "Silly fool" , "Ninny" , "Tease - fool" .)