Succulent tuna finally donated (5)
I believe the answer is:
agave
'succulent' is the definition.
(I've seen this before)
'tuna finally donated' is the wordplay.
'finally' says to take the final letters.
'donated' becomes 'gave' (give can mean to donate).
The last letter of 'tuna' is 'a'.
'a'+'gave'='AGAVE'
(Other definitions for agave that I've seen before include "Plant, source of sisal" , "Succulent; daughter of Cadmus" , "A cactus" , "native growing up in tropical America" , "Succulent plant" .)