Ruin holiday (5)
I believe the answer is:
break
This clue is a double definition.
'ruin' is the first definition.
(breaking is a kind of ruining)
'holiday' is the second definition.
(thesaurus)
(Other definitions for break that I've seen before include "Baker (anag.)" , "Short holiday" , "accumulation of points" , "Holiday; decipher (code)" , "Damage - interrupt" .)