Opening within chain letter (5)
I believe the answer is:
inlet
'opening' is the definition.
(I've seen this before)
'within chain letter' is the wordplay.
'within' indicates the answer is hidden within the clue.
'INLET' is hidden within 'chain letter'.
(Other definitions for inlet that I've seen before include "Means of entry" , "Bay or recess" , "Entry; arm of the sea" , "bit of a lake here?" , "Mouth of creek" .)