Some Bantu besotted with thermionic valves (5)
I believe the answer is:
tubes
'thermionic valves' is the definition.
('tube' can be a synonym of 'thermionic valve')
'some bantu besotted with' is the wordplay.
'some' indicates the answer is hidden within the clue (some parts of the word are used).
'TUBES' can be found hidden inside 'bantu besotted with'.
(Other definitions for tubes that I've seen before include "Long hollow conduits" , "Hollow cylindrical shapes" , "Long hollow cylinders" , "Piping" , "Hoses, pipes, lines etc" .)