It's not simple to make up (8)
I believe the answer is:
compound
This clue is a double definition.
'it's not simple' is the first definition.
('compound' is the opposite to 'simple')
'make up' is the second definition.
(I've seen this before)
'to' is the link.
(Other definitions for compound that I've seen before include "fenced area in prison" , "Composed of two or more separate elements" , "Exacerbate - enclosure" , "Chemical substance; enclosed space" , "Make worse; property enclosure" .)