Currency in which dude invested somewhere in Mexico (7)
I believe the answer is:
yucatan
'somewhere in mexico' is the definition.
(state of Mexico)
'currency in which dude' is the wordplay.
'currency' becomes 'yuan' (currency of China).
'in which' is an insertion indicator.
'dude' becomes 'cat' (American slang for man, esp in Jazz).
'yuan' going around 'cat' is 'YUCATAN'.
'invested' acts as a link.
I am not very happy about this link. It may be part of another bit of the clue.
(Other definitions for yucatan that I've seen before include "Mayan area of Mexico" , "Region of Mexico" , "Alcoholic drink" , "part of Mexico" , "Mexican state" .)