Ditch is dry, empty and retained with no point (4)
I believe the answer is:
dyke
'ditch' is the definition.
(I've seen this before)
'dry empty and retained with no point' is the wordplay.
'empty' says to hollow out the word (remove centre letters).
'and' is a charade indicator (letters next to each other).
'retained with no point' becomes 'ke' (I can't justify this - if you can you should believe this answer much more).
'dry' with its centre taken out is 'dy'.
'dy'+'ke'='DYKE'
'is' is the link.
Can you help me to learn more?
(Other definitions for dyke that I've seen before include "Offa's barrier?" , "Barrier to keep out the sea" , "Earthwork; watercourse" , "Flood barrier" , "Flood defence" .)