Plain fool having chart reversed (6)
I believe the answer is:
pampas
'plain' is the definition.
(I've seen this before)
'fool having chart reversed' is the wordplay.
'fool' becomes 'sap' (synonyms).
'having' means one lot of letters go next to another.
'chart' becomes 'map' (synonyms).
'reversed' says the letters should be written in reverse.
'sap'+'map'='sapmap'
'sapmap' reversed gives 'PAMPAS'.
(Other definitions for pampas that I've seen before include "Argentina grassy plain" , "Grassy plains of South America" , "7" , "Prairies (with grass)" , "Grassy plains of Argentine" .)