Lead on diamonds, then duck? (6)
I believe the answer is:
delude
'lead on' is the definition.
('delude' can be a synonym of 'lead on')
'diamonds then duck?' is the wordplay.
'diamonds' becomes 'd' (abbreviation in bridge say).
'then' says to put letters next to each other.
'duck?' becomes 'elude' ('elude' can be a synonym of 'duck'**).
'd'+'elude'='DELUDE'
(Other definitions for delude that I've seen before include "Mislead in belief" , "Mislead, beguile" , "Cause to believe a lie" , "Fool, deceive" , "Lead into false belief" .)