Mercy from Lenin admitting end of regime on country borders (8)
I believe the answer is:
leniency
'mercy' is the definition.
(leniency is a kind of mercy)
'from lenin admitting end of regime on country borders' is the wordplay.
'from' is an anagram indicator (from the letters).
'admitting' indicates putting letters inside.
'end of' says to take the final letters.
'on' means one lot of letters go next to another.
'borders' suggests removing the centre (only the edge or border letters remain).
'country' with its centre removed is 'cy'.
The last letter of 'regime' is 'e'.
'lenin' anagrammed gives 'lnien'.
'lnien' placed around 'e' is 'lenien'.
'lenien'+'cy'='LENIENCY'
(Other definitions for leniency that I've seen before include "Mercifulness" , "Clemency, mercy" , "Opposite of severity - state of mildness" , "Mildness and tolerance" , "Forbearance" .)