Later addition making contribution to rafter, though temporary (12)
I believe the answer is:
afterthought
'later addition' is the definition.
(I've seen this before)
'contribution to rafter though temporary' is the wordplay.
'contribution to' indicates the answer is hidden within the clue.
'AFTERTHOUGHT' is hidden in the letters of 'rafter though temporary'.
'making' acts as a link.
(Other definitions for afterthought that I've seen before include "not an original idea" , "Re-evaluation" , "PS for instance" , "Later addition" , "Something added later" .)