The reason he excluded serious crime (7)
I believe the answer is:
treason
'serious crime' is the definition.
(I've seen this before)
'the reason he excluded' is the wordplay.
'excluded' is a deletion indicator.
'the'+'reason'='thereason'
'thereason' with 'he' taken away is 'TREASON'.
(Other definitions for treason that I've seen before include "Crime of disloyalty" , "Perfidy" , "Betrayal of one's nation" , "Offence against state" , "Deliberate act of betrayal" .)