Partly fill an order that's plain (5)
I believe the answer is:
llano
'plain' is the definition.
(thesaurus)
'partly fill an order' is the wordplay.
'partly' says the answer is hidden in the clue.
'LLANO' is hidden within 'fill an order'.
'that's' is the link.
(Other definitions for llano that I've seen before include "(South American) grassy plain" , "Treeless grassy plain in South America" , "Grassy plain in South?America" , "Treeless area" , "Flat area" .)