A whole lot of mothers and one son (4)
I believe the answer is:
mass
'a whole lot' is the definition.
('mass' can be a synonym of 'whole lot')
'mothers and one son' is the wordplay.
'mothers' becomes 'mas' (ma is a kind of mother).
'and' says to put letters next to each other.
'one son' becomes 's' (genealogical abbreviation for son).
'mas'+'s'='MASS'
'of' is the link.
(Other definitions for mass that I've seen before include "Religious ceremony" , "seasonal midnight service?" , "state, briefly" , "Holy service" , "Pound, maybe" .)