Queen back to get eastern king to hum (4)
I believe the answer is:
reek
'hum' is the definition.
(thesaurus)
'queen back to get eastern king' is the wordplay.
'queen' becomes 'er' (abbreviation for Elizabeth Regina).
'back' says the letters should be written backwards.
'to get' means one lot of letters go next to another.
'eastern' becomes 'e'.
'king' becomes 'k'.
'er' written backwards gives 're'.
're'+'e'+'k'='REEK'
'to' is the link.
(Other definitions for reek that I've seen before include "Smell pungently" , "Strong smell" , "Smell bad" , "Smell strongly" , "Smell like 12 Across" .)