Centres of gas planets ultimately aligned in conjunction (3)
I believe the answer is:
and
'conjunction' is the definition.
('and' is a conjunction)
'centres of gas planets ultimately aligned' is the wordplay.
'centres of gas planets' becomes 'an' (I can't explain this - if you can you should give a lot more credence to this answer).
'ultimately' suggests the final letters.
The final letter of 'aligned' is 'd'.
'an'+'d'='AND'
'in' acts as a link.
(Other definitions for and that I've seen before include "Joining word" , "See 20" , "Joiner or dandies" , "It connects words, clauses or sentences" , "Together with" .)