In old money, wood is scary (5-8)
I believe the answer is:
spine-chilling
'scary' is the definition.
(I've seen this before)
'in old money wood' is the subsidiary indication.
'in' indicates putting letters inside.
'old money' becomes 'schilling'.
'wood' becomes 'pine' (pine is a kind of wood).
'schilling' enclosing 'pine' is 'spinechilling'.
'is' is the link.
(Other definitions for spine-chilling that I've seen before include "Inspiring both terror and excitement" , "Arousing terror" , "Bloodcurdling" , "Scary" , "Terrifying" .)