Mike invested in fuel — that’s ridiculous (7)
I believe the answer is:
comical
'ridiculous' is the definition.
(thesaurus)
'mike invested in fuel' is the wordplay.
'mike' becomes 'mic' (I've seen this in other clues).
'invested in' means one lot of letters goes inside another.
'fuel' becomes 'coal' (coal is a kind of fuel).
'mic' placed within 'coal' is 'COMICAL'.
'that's' is the link.
(Other definitions for comical that I've seen before include "Causing laughter" , "Rich" , "Amusing or droll" , "Witty" , "priceless" .)