Tricks Methuselah will use shortly ... is one briefly taken in? (9)

'tricks' is the definition.
('illusion' can be a synonym of 'trick')

'methuselah will use shortly is one briefly taken in?' is the wordplay.
'methuselah will' becomes 'ill' (where the crossword setter is 'Methuselah').
'shortly' means to remove the last letter.
'briefly' means to remove the last letter.
'taken in?' indicates putting letters inside.
'use' with its final letter taken off is 'us'.
'one' with its final letter taken away is 'on'.
'is' going around 'on' is 'ions'.
'ill'+'us'+'ions'='ILLUSIONS'