Most senior ministers throw out probes about alien visitor (7)
I believe the answer is:
cabinet
'most senior ministers' is the definition.
'throw out probes about alien visitor' is the wordplay.
'throw out' becomes 'bin'.
'probes' means one lot of letters goes inside another.
'about' becomes 'ca' (abbreviation for 'circa'**).
'alien visitor' becomes 'et' (extraterrestrial).
'ca'+'et'='caet'
'bin' put within 'caet' is 'CABINET'.
(Other definitions for cabinet that I've seen before include "Senior government ministers" , "Government committee" , "Case" , "elected ministers" , "some PMs dislike?" .)