Custom I'd picked up in train to Barking (9)
I believe the answer is:
tradition
'custom' is the definition.
(I know that custom can be written as tradition)
'i'd picked up in train to barking' is the wordplay.
'picked up' says the letters should be written in reverse.
'in' is an insertion indicator.
'barking' indicates anagramming the letters ('barking' can informally mean mad).
'id' written backwards gives 'di'.
'train'+'to'='trainto'
'trainto' is an anagram of 'tration'.
'di' put into 'tration' is 'TRADITION'.
(Other definitions for tradition that I've seen before include "Body of beliefs and customs handed down through generations" , "Inherited custom" , "customary behaviour" , "stuck in the old ways" , "Beliefs, etc, handed down" .)