The opening of a chain letter (5)
I believe the answer is:
inlet
'the opening' is the definition.
(inlet can mean a small opening)
'of a chain letter' is the wordplay.
'of' indicates the answer is hidden within the clue.
'INLET' can be found hidden inside 'a chain letter'.
(Other definitions for inlet that I've seen before include "Small bay or creek" , "Bay or recess" , "Small coastal opening" , "Arm off a larger body of water between headlands" , "Coastal feature" .)