Forbidden fruit -- finally getting a raspberry! (5)
I believe the answer is:
taboo
'forbidden' is the definition.
('taboo' can be a synonym of 'forbidden')
'fruit finally getting a raspberry' is the wordplay.
'finally' suggests the final letters.
'getting' means one lot of letters go next to another.
'raspberry' becomes 'boo' ('boo' can be a synonym of 'raspberry'**).
The final letter of 'fruit' is 't'.
't'+'a'+'boo'='TABOO'
(Other definitions for taboo that I've seen before include "Ritually forbidden" , "ruled out" , "No-no" , "Prohibited by social custom" , "Strictly forbidden" .)