Unhappiness of Scrooge followed by joy at last (6)
I believe the answer is:
misery
'unhappiness' is the definition.
(misery is a kind of unhappiness)
'scrooge followed by joy at last' is the wordplay.
'scrooge' becomes 'miser' (synonyms).
'followed by' says to put letters next to each other.
'at last' indicates one should take the final letters.
The final letter of 'joy' is 'y'.
'miser'+'y'='MISERY'
'of' acts as a link.
(Other definitions for misery that I've seen before include "Feeling of great unhappiness" , "Distress, wretchedness" , "Woe, wretchedness" , "Intense unhappiness" , "Wretchedness, suffering" .)