Gull in delta with duck (6)
I believe the answer is:
delude
'gull' is the definition.
(I know that gull is a more specific form of the action delude)
'delta with duck' is the wordplay.
'delta' becomes 'd' (phonetic alphabet: alpha, bravo, charlie etc.).
'with' is a charade indicator (letters next to each other).
'duck' becomes 'elude' (synonyms).
'd'+'elude'='DELUDE'
'in' is the link.
(Other definitions for delude that I've seen before include "Eluded so as to deceive" , "Mislead; deceive" , "Deceive, lead on" , "Lead into false belief" , "Cause to believe something untrue, deceive" .)