Small steps in emissions, ultimately for Musk's costliest vehicles (10)

'vehicles' is the definition.
(spaceship is a kind of vehicle)

'small steps in emissions ultimately' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'steps' becomes 'paces' (pace is a kind of step).
'in' becomes 'hip' (both can mean fashionable).
'ultimately' suggests the final letters.
The last letter of 'emissions' is 's'.
's'+'paces'+'hip'+'s'='SPACESHIPS'

'for musk's costliest' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.

Can you help me to learn more?