Cost nearly something astronomical (5)
I believe the answer is:
comet
'something astronomical' is the definition.
I can't judge whether this defines the answer.
'cost nearly' is the wordplay.
'cost' becomes 'come to' (I've seen this before).
'nearly' means to remove the last letter.
'cometo' with its final letter removed is 'COMET'.
Can you help me to learn more?
(Other definitions for comet that I've seen before include "Heavenly visitor" , "Rudolph's companion" , "Dancer's colleague" , "Heavenly body with a fiery tail" , "Halley's space traveller?" .)