Main reason corrupt Reagan's deposed? (3)
I believe the answer is:
sea
'main' is the definition.
(the main is an old name for the sea)
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for sea that I've seen before include "The deep" , "one may be pounding the beach" , "Expanse of salt water" , "Ocean area" , "Lesser ocean" .)