Personally returned some of them, that’s plain (5)
I believe the answer is:
llano
'that's plain' is the definition.
(a grassy plain)
'personally returned some of' is the wordplay.
'some of' indicates the answer is hidden within the clue.
'returned' says the letters should be written in reverse.
'onall' can be found hidden inside 'personally'.
'onall' backwards is 'LLANO'.
'them' acts as a link.
This may not be right. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for llano that I've seen before include "(South American) grassy plain" , "Treeless grassy plain in South America" , "Flat area" , "Grassy plain in South?America" , "Treeless area" .)