Are set to form a union in: it's simple (7)
I believe the answer is:
austere
'simple' is the definition.
(I've seen this before)
'are set to form a union in' is the wordplay.
'to form' indicates anagramming the letters.
'a union' becomes 'u' (common abbreviation as in 'Trades Union Congress').
'in' is an insertion indicator.
'are'+'set'='areset'
'areset' anagrammed gives 'astere'.
'astere' going around 'u' is 'AUSTERE'.
'it's' is the link.
(Other definitions for austere that I've seen before include "Severe, ascetic" , "Severe or strict in attitude" , "Hard" , "Severe in manner" , "Stark" .)