Initially joined alien aircraft (3)
I believe the answer is:
jet
'aircraft' is the definition.
(jet is a kind of aircraft)
'initially joined alien' is the wordplay.
'initially' says to take the initial letters.
'alien' becomes 'et' (extra terrestrial).
The initial letter of 'joined' is 'j'.
'j'+'et'='JET'
(Other definitions for jet that I've seen before include "Black variety of lignite, used for ornaments" , "Deep black" , "The blackest black" , "Rapid stream of liquid or gas" , "Black aircraft?" .)