Serious crime: leader denied motive (6)
I believe the answer is:
reason
'motive' is the definition.
(I know that reason is a type of rational motive)
'serious crime leader denied' is the wordplay.
'serious crime' becomes 'treason' (treason is a crime).
'leader denied' means to remove the first letter.
'treason' with its initial letter taken off is 'REASON'.
(Other definitions for reason that I've seen before include "Motive; argue" , "Cause, explanation" , "Rational motive" , "Sanity of senora" , "Cause - logic" .)