Opening of chain letter (5)
I believe the answer is:
inlet
'opening' is the definition.
(I've seen this before)
'of chain letter' is the wordplay.
'of' says the answer is hidden in the clue.
'INLET' is hidden in the letters of 'chain letter'.
(Other definitions for inlet that I've seen before include "Small opening of coastline" , "Mouth of creek" , "An entrance or opening" , "Bay, cove" , "Bay, bight" .)