Somewhere in Belgium is missing an idiot (5)
I believe the answer is:
twerp
'idiot' is the definition.
(I've seen this before)
'somewhere in belgium is missing an' is the wordplay.
'somewhere in belgium' becomes 'antwerp' (Belgian city).
'is missing' is a deletion indicator.
'antwerp' with 'an' taken out is 'TWERP'.
(Other definitions for twerp that I've seen before include "Twit" , "Nitwit" , "Numpty" , "Pillock" , "Ninny" .)