They’re regularly lacking finish (8)
I believe the answer is:
terminus
'finish' is the definition.
(synonyms)
'they're regularly lacking' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'lacking' becomes 'minus' (I've seen this in another clue).
The alternate letters of 'theyre' are 'ter'.
'ter'+'minus'='TERMINUS'
(Other definitions for terminus that I've seen before include "final point" , "Final station on bus route" , "End of the line, bus or train" , "journey's end" , "End - objective" .)