Piano not in time upset sweetheart (5)
I believe the answer is:
petal
'time upset sweetheart' is the definition.
Although both the answer and definition are singular nouns, I don't see how one could define the other.
'piano not' is the wordplay.
'piano' becomes 'p'.
'not' becomes 'etal' (I am not sure about this - if you are sure you should believe this answer much more).
'p'+'etal'='PETAL'
'in' is the link.
Can you help me to learn more?
(Other definitions for petal that I've seen before include "One piece of corolla" , "Corolla leaf" , "Floral segment" , "Coloured segment of flower" , "Flower part seen on plate" .)