Swear, “Grain gets hot” (4)
I believe the answer is:
oath
'swear' is the definition.
(thesaurus)
'grain gets hot' is the wordplay.
'grain' becomes 'oat' (oat is a kind of grain).
'gets' means one lot of letters go next to another.
'hot' becomes 'h' (abbreviation eg on taps).
'oat'+'h'='OATH'
(Other definitions for oath that I've seen before include "swearing" , "Vow or expletive" , "Curse or solemn word" , "exasperated comment" , "Profane or obscene expression" .)