Masses from the Deep South (6)
I believe the answer is:
oceans
'masses' is the definition.
(ocean can mean a mass or large amount of something)
'the deep south' is the wordplay.
'the deep' becomes 'ocean' (I've seen this before).
'south' becomes 's' (abbreviation).
'ocean'+'s'='OCEANS'
'from' acts as a link.
(Other definitions for oceans that I've seen before include "Vast bodies of water" , "big seas" , "Great seas" , "Large seas" , "Bodies of salt water" .)