Key I have to donate (4)
I believe the answer is:
give
'to donate' is the definition.
(I've seen this before)
'key i have' is the wordplay.
'key' becomes 'g' (musical key).
'i have' becomes 'I've' (contraction of 'I have').
'g'+'ive'='GIVE'
(Other definitions for give that I've seen before include "Donate, grant" , "Offer, elasticity" , "Yield, produce" , "be generous?" , "6" .)