Splitting the atom in a ditch (4)
I believe the answer is:
moat
'ditch' is the definition.
(I know that moat is a type of trench)
'splitting the atom' is the wordplay.
'splitting' indicates an anagram (I've seen 'split' mean this (letters split up and reassembled)).
'atom' with letters rearranged gives 'MOAT'.
'in a' acts as a link.
(Other definitions for moat that I've seen before include "defensive construction" , "Castle's ditch" , "Trench round a castle" , "A deep water-filled ditch around a castle" , "Water-filled trench around a castle" .)