It's any made-up reason (6)
I believe the answer is:
sanity
'reason' is the definition.
(thesaurus)
'it's any made-up' is the wordplay.
'made up' is an anagram indicator.
'its'+'any'='itsany'
'itsany' is an anagram of 'SANITY'.
(Other definitions for sanity that I've seen before include "Stay in (anag.)" , "Soundness of mind" , "Condition of having a sound mind" , "Mental normality" , "Sound powers of mind" .)