They're known for the speed of their heavenly careers (7)
I believe the answer is:
meteors
'heavenly careers' is the definition.
Although both the answer and definition are plural nouns, I can't see how one could define the other.
'they're known for the speed of' is the wordplay.
I cannot quite understand how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found within the answer.
'of' could be 'o' and 'o' is present in the answer.
The remaining letters 'meers' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'their' is the link.
Can you help me to learn more?
(Other definitions for meteors that I've seen before include "Shooting stars" , "Space fireballs" .)