I'm aching to swim lake (8)
I believe the answer is:
'lake' is the definition.
(I know that Michigan is a lake)
'i'm aching to swim' is the wordplay.
'to swim' indicates anagramming the letters.
'imaching' is an anagram of 'MICHIGAN'.
(Other definitions for michigan that I've seen before include "State of Detroit" , "I'm aching to go out the US state" , "US state and Great Lake" , "Lake by which Chicago stands" , "I'm aching to see American state" .)