Fantasies about mothers overwhelming (6)
I believe the answer is:
'fantasies' is the definition.
(dream is a kind of fantasy)
'about mothers overwhelming' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'mothers' becomes 'dams' (dam can mean the mother of an animal).
'overwhelming' is an insertion indicator.
're' placed into 'dams' is 'DREAMS'.
(Other definitions for dreams that I've seen before include "Imaginings" , "Fantasies; fantasises" , "Reveries or cherished aspirations" , "Fantasy ambitions" , "Fancies" .)