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 a hidden word.
'MAR' can be found hidden inside 'denmark'.
(Other definitions for mar that I've seen before include "Ram into and spoil" , "Spanish main?" , "Cause harm" , "'Damage, impair (3)'" , "Make imperfect" .)