Brilliant travellers encountered in small firms (6)
I believe the answer is:
comets
'brilliant travellers' is the definition.
Although both the answer and definition are plural nouns, I don't see how one could define the other.
'encountered in small firms' is the wordplay.
'encountered' becomes 'met' ('meet' can be a synonym of 'encounter').
'in' is an insertion indicator.
'small firms' becomes 'cos' (co is short for company - a business or firm; 'small' indicates an abbreviation).
'met' inserted into 'cos' is 'COMETS'.
Can you help me to learn more?
(Other definitions for comets that I've seen before include "Space travellers" , "Heavenly bodies with tails" , "Small extraterrestrial bodies" .)