Some regret naming a border town (6)
I believe the answer is:
'town' is the definition.
(town in Scotland)
'some regret naming a border' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'GRETNA' can be found hidden inside 'regret naming border'.
(Other definitions for gretna that I've seen before include "Green for eloping couples" , "... Green, for elopers" , "Scottish town" , "border village" .)