Ruin in Denmark (3)
I believe the answer is:
mar
'ruin' is the definition.
(I've seen this before)
'in denmark' is the wordplay.
'in' indicates the answer is hidden within the clue.
'MAR' is hidden in the letters of 'denmark'.
(Other definitions for mar that I've seen before include "Ram into and spoil" , "Spanish main?" , "Cause harm" , "'Damage, impair (3)'" , "Make imperfect" .)