Plain girl father initially supported (6)
I believe the answer is:
pampas
'plain' is the definition.
(thesaurus)
'girl father initially supported' is the wordplay.
'girl' becomes 'pam' (**).
'father' becomes 'pa' (pa is a kind of father).
'initially' suggests taking the first letters.
The initial letter of 'supported' is 's'.
'pam'+'pa'+'s'='PAMPAS'
(Other definitions for pampas that I've seen before include "Argentina grassy plain" , "South American plain" , "Grassy plains of South America" , "Argentinian plains" , "South American grassland" .)