Plain little son and a very little girl (7)
I believe the answer is:
savanna
'plain' is the definition.
(I've seen this before)
'little son and a very little girl' is the wordplay.
'little son' becomes 's' (abbreviation for son).
'and' means one lot of letters go next to another.
'very little' becomes 'v' (abbreviation or little form for 'very').
'girl' becomes 'anna' (**).
's'+'a'+'v'+'anna'='SAVANNA'
(Other definitions for savanna that I've seen before include "Tropical African grasslands" , "Grassy (African) plain" , "African grassland" , "Open grasslands of Africa" , "Large grassy plain with few trees" .)