Plant, end of stem becoming black, declined (5)
I believe the answer is:
ebbed
'declined' is the definition.
(I've seen this in another clue)
'plant end of stem becoming black' is the wordplay.
I cannot really understand how this works, but
'black' could be 'bb' and 'bb' is located in the answer.
The remaining letters 'eed' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for ebbed that I've seen before include "Diminished" , "Flowed back or receded" , "Went out like tide" .)