A lack of agreement in retreat (4)
I believe the answer is:
nook
'retreat' is the definition.
(I know that nook is a type of retreat)
'a lack of agreement' is the wordplay.
'a lack of' becomes 'no' (lack of money is no money).
'agreement' becomes 'ok' (I've seen this in other clues).
'no'+'ok'='NOOK'
'in' acts as a link.
(Other definitions for nook that I've seen before include "Recess or cranny" , "Interior angle formed where two walls meet" , "Recess offering seclusion" , "Alcove" , "Small space" .)