Because I'm travelling skywards, the speed of light is enormous (6)
I believe the answer is:
cosmic
'is enormous' is the definition.
(I've seen this before)
'because i'm travelling skywards' is the wordplay.
'because' becomes 'cos' (I've seen this in other clues).
'travelling' is an anagram indicator.
'skywards' becomes 'c' (I can't justify this - if you can you should believe this answer much more).
'im' with letters rearranged gives 'mi'.
'cos'+'mi'+'c'='COSMIC'
'the speed of light' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for cosmic that I've seen before include "Originating in outer space" , "Large -- universal" , "To do with the whole universe" , "Grandiose" , "Of the universe, infinite" .)