Regrets penning last of chain letters (5)
I believe the answer is:
runes
'letters' is the definition.
(I know this)
'regrets penning last of chain' is the wordplay.
'regrets' becomes 'rues' ('rue' can be a synonym of 'regret').
'penning' indicates putting letters inside.
'last of' suggests the final letters.
The last letter of 'chain' is 'n'.
'rues' going around 'n' is 'RUNES'.
(Other definitions for runes that I've seen before include "Small stones bearing symbols with magic significance" , "Old letters" , "Mysterious letters" , "marks on old tablets" , "old characters" .)