Even in prison, Donald's heading for president (9)
I believe the answer is:
cleveland
'for president' is the definition.
(Grover Cleveland)
'even in prison donald's heading' is the wordplay.
'even' becomes 'level' (similar in meaning).
'in' is an insertion indicator.
'prison' becomes 'can' (the can is American slang for prison).
'heading' suggests taking the first letters (head could mean the top letter).
The initial letter of 'donalds' is 'd'.
'level' going into 'can' is 'clevelan'.
'clevelan'+'d'='CLEVELAND'
(Other definitions for cleveland that I've seen before include "past president" , "Region of NE England" , "former President" , "ex-President" , "Northeast English county, created 1974" .)