Act deceitfully as sea god, keeping close to enemy (3,2,2)
I believe the answer is:
try it on
'act deceitfully' is the definition.
'sea god keeping close to enemy' is the wordplay.
'sea god' becomes 'triton' (I've seen this before).
'keeping' is an insertion indicator.
'close to' says to take the final letters (the close/ending of).
The final letter of 'enemy' is 'y'.
'triton' enclosing 'y' is 'TRY IT ON'.
'as' is the link.
(Other definitions for try it on that I've seen before include "Aim to deceive" , "practise a little deception" , "Attempt a fitting" , "See how far one may go" , "Attempt to be provocative (colloq.)" .)