Terribly discontented, vandalise railway (4)
I believe the answer is:
very
'terribly' is the definition.
(I've seen this in another clue)
'discontented vandalise railway' is the wordplay.
'discontented' says to hollow out the word (remove centre letters) (the word's contents are removed).
'railway' becomes 'ry' (abbreviation).
'vandalise' with its centre taken out is 've'.
've'+'ry'='VERY'
(Other definitions for very that I've seen before include "Frightfully" , "Extremely, in a high degree" , "light, perhaps" , "Sort of light" , "To a large extent" .)