A stamp used initially for a farewell (5)
I believe the answer is:
adieu
'for a farewell' is the definition.
(adieu is a kind of farewell)
'a stamp used initially' is the wordplay.
'stamp' becomes 'die' (I've seen this in other clues).
'initially' suggests taking the first letters.
The initial letter of 'used' is 'u'.
'a'+'die'+'u'='ADIEU'
(Other definitions for adieu that I've seen before include "Parting greeting" , "Au revoir (to a kind friend?)" , "left, making this?" , "Gallic goodbye" , "Farewell, adios" .)