Father and son crossing first of many treeless plains (6)
I believe the answer is:
pampas
'treeless plains' is the definition.
(I've seen this before)
'father and son crossing first of many' is the wordplay.
'father' becomes 'papa' (papa can mean one's father).
'and' means one lot of letters go next to another.
'son' becomes 's' (genealogical abbreviation for son).
'crossing' means one lot of letters goes inside another (to cross can mean to go over or around).
'first of many' becomes 'm' (1st letter of 'many').
'papa'+'s'='papas'
'papas' going around 'm' is 'PAMPAS'.
(Other definitions for pampas that I've seen before include "South American plain" , "Treeless plain(s)" , "Great plains of Argentina" , "Great plains of S America (6)" , "part of South America" .)