Setter repeatedly enthusiastic about unopened souvenir (7)
I believe the answer is:
memento
'souvenir' is the definition.
('memento' can be a synonym of 'souvenir')
'setter repeatedly enthusiastic about unopened' is the wordplay.
'setter' becomes 'me' (the setter of the clue).
'repeatedly' indicates the doubling of some letters.
'enthusiastic about' becomes 'into' (I've seen this before).
'unopened' means to remove the first letter.
'me' doubled is 'meme'.
'into' with its first letter taken away is 'nto'.
'meme'+'nto'='MEMENTO'
(Other definitions for memento that I've seen before include "Souvenir, reminder of past event" , "Something kept" , "Object kept as a souvenir" , "Reminder that death is inevitable" , "Souvenir, something to remember by" .)