Bumbling train thief with multiple convictions (10)
I believe the answer is:
interfaith
'with multiple convictions' is the definition.
(conviction can refer to faith)
'bumbling train thief' is the wordplay.
'bumbling' indicates an anagram.
'train'+'thief'='trainthief'
'trainthief' with letters rearranged gives 'INTERFAITH'.
(Other definitions for interfaith that I've seen before include "With various religions" , "Like ecumenical event" .)