Soundly gives a rich aspect to certain companies (6)
I believe the answer is:
guilds
'soundly gives a rich aspect' is the definition.
I can't judge whether this definition defines the answer.
'certain companies' is the wordplay.
'certain' becomes 'gui' (I can't explain this - if you can you should believe this answer much more).
'companies' becomes 'Ld.s' (Ld. is an example).
'gui'+'lds'='GUILDS'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for guilds that I've seen before include "Craftsmen's associations" , "Associations of craftsmen in Middle Ages" , "Clubs, lodges" , "Associations fo people with common interests or purposes" .)