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 don't understand how they can define each 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 in the answer.
'of' could be 'o' and 'o' is found within 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' acts as a link.
Can you help me to learn more?
(Other definitions for meteors that I've seen before include "Shooting stars" , "Space fireballs" .)