H and M in America are drones (4)
I believe the answer is:
hums
'are drones' is the definition.
(I've seen this in another clue)
'h and m in america' is the wordplay.
'and' says to put letters next to each other.
'in' indicates putting letters inside.
'america' becomes 'us' (United States).
'm' going into 'us' is 'ums'.
'h'+'ums'='HUMS'
(Other definitions for hums that I've seen before include "Sings with closed lips" , "Sings wordlessly" .)