First character donated plant (5)
I believe the answer is:
'plant' is the definition.
(agave is a kind of plant)
'first character donated' is the wordplay.
'first' is a charade indicator (letters next to each other).
'character' becomes 'a'.
'donated' becomes 'gave' (I've seen this before).
'a' put next to 'gave' is 'AGAVE'.
(Other definitions for agave that I've seen before include "Plant, source of sisal", "plant in Mexico", "Spiky succulent", "source of tequila", "Succulent plant".)