It's hot for them at the border (3)
I believe the answer is:
hem
'the border' is the definition.
(thesaurus)
'it's hot for them' is the wordplay.
'it's hot' becomes 'h'.
'for' is a charade indicator (letters next to each other).
'them' becomes 'em'.
'h'+'em'='HEM'
'at' acts as a link.
(Other definitions for hem that I've seen before include "Double-backed edge" , "Part of skirt" , "A stitched border of cloth" , "Bottom of a garment" , "Express hesitation" .)