Bucks laid out, investing in gems mostly, or toy design (6,4)

'toy design' is the definition.
(Rubik's cube is a toy)

'bucks laid out investing in gems mostly' is the wordplay.
'laid out' is an anagram indicator.
'investing in' is an insertion indicator.
'gems' becomes 'rubies' (ruby is a kind of gem).
'mostly' means to remove the last letter.
'bucks' is an anagram of 'kscub'.
'rubies' with its final letter taken off is 'rubie'.
'kscub' placed within 'rubie' is 'RUBIKS CUBE'.

'or' acts as a link.