Opening part of chain letter (5)
I believe the answer is:
inlet
'opening' is the definition.
(inlet can mean a small opening)
'part of chain letter' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'INLET' can be found hidden inside 'chain letter'.
(Other definitions for inlet that I've seen before include "Arm of lake, river" , "Place of entry" , "Entry port" , "Narrow arm of water" , "Arm off a larger body of water between headlands" .)