Broadcaster’s black Hummer heading west (4)
I believe the answer is:
beeb
'broadcaster's' is the definition.
(I've seen this before)
'black hummer heading west' is the wordplay.
'black' becomes 'b' (abbreviation as in pencil hardness).
'hummer' becomes 'bee'.
'heading west' says the letters should be written backwards.
'bee' written backwards gives 'eeb'.
'b'+'eeb'='BEEB'
(Other definitions for beeb that I've seen before include "British Broadcasting Corporation (popularly)" , "BBC, familiarly" , "BBC (informally)" , "Old microcomputer" , "BBC (colloq" .)