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' says the answer is hidden in the clue.
'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" .)