Opening some chain letters (5)
I believe the answer is:
inlet
'opening' is the definition.
(inlet can mean a small opening)
'some chain letters' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'INLET' is hidden within 'chain letters'.
(Other definitions for inlet that I've seen before include "Arm off a larger body of water between headlands" , "Bay or recess" , "access point" , "Strip of water extending from sea into land" , "Arm of lake, river" .)