Sea god assuming tail of ray in attempt to deceive (3,2,2)
I believe the answer is:
try it on
'attempt to deceive' is the definition.
(I've seen this before)
'sea god assuming tail of ray' is the wordplay.
'sea god' becomes 'triton' (figure of Greek myth).
'assuming' means one lot of letters goes inside another.
'tail of' indicates one should take the final letters.
The final letter of 'ray' is 'y'.
'triton' going around 'y' is 'TRY IT ON'.
'in' is the link.
(Other definitions for try it on that I've seen before include "Push one's luck" , "When buying new suit one should" , "Test the boundaries" , "be bold" , "Attempt to be provocative (colloq.)" .)