False alibi initially excluded by magistrate (6)

'magistrate' is the definition.
(I have seen 'Scottish magistrate' mean 'bailie' so perhaps 'magistrate' could also mean 'bailie')

'false alibi initially excluded' is the wordplay.
'false' indicates anagramming the letters.
'initially' says to take the initial letters.
The initial letter of 'excluded' is 'e'.
'alibi' with letters rearranged gives 'baili'.
'baili'+'e'='BAILIE'

'by' acts as a link.