Orderly system in discos mostly (6)
I believe the answer is:
cosmos
'orderly system' is the definition.
'in discos mostly' is the wordplay.
'in' indicates a hidden word.
'COSMOS' is hidden within 'discos mostly'.
(Other definitions for cosmos that I've seen before include "Universe as an ordered system" , "Universe as an ordered whole" , "Everything that exists" , "The entire universe." , "well-organised world" .)