Union of College with High that’s illegal here (8)
I believe the answer is:
polygamy
'that's illegal here' is the definition.
(bigamy or polygamy is a crime)
'union of college with high' is the wordplay.
'union of college' becomes 'poly' (I can't justify this - if you can you should believe this answer much more).
'with' means one lot of letters go next to another.
'high' becomes 'gamy' (synonyms).
'poly'+'gamy'='POLYGAMY'
Can you help me to learn more?
(Other definitions for polygamy that I've seen before include "excessively high number of matches" , "having several mates" , "Multiple marriage condition" , "Multiple mating" , "Having many wives" .)