Excessive unions oddly over America supporting Republican (7)
I believe the answer is:
ruinous
'excessive' is the definition.
Both the answer and definition are adjectives. Perhaps they are linked in a way I don't understand?
'unions oddly over america supporting republican' is the wordplay.
'oddly' means one should take alternating letters.
'over america' becomes 'ous' (I can't explain this - if you can you should give a lot more credence to this answer).
'supporting' means one lot of letters go next to another (in a down clue, some letters go below others).
'republican' becomes 'r' (abbreviation).
The alternating letters of 'unions' are 'uin'.
'uin'+'ous'='uinous'
'uinous' after 'r' is 'RUINOUS'.
Can you help me to learn more?
(Other definitions for ruinous that I've seen before include "far too expensive" , "Disastrous, destructive" , "Broken down" , "causing harm?" , "Destructive and causing downfall" .)