Firm encountered a celestial object (5)
I believe the answer is:
comet
'a celestial object' is the definition.
(I've seen this before)
'firm encountered' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'encountered' becomes 'met' ('meet' can be a synonym of 'encounter').
'co'+'met'='COMET'
(Other definitions for comet that I've seen before include "one of Santa's reindeer" , "Frozen body" , "astronomical sight" , "traveller covering huge mileage" , "Heavenly body with a fiery tail" .)