Upset that key is removed from lock (5)
I believe the answer is:
tress
This could be a double definition.
'upset that key is removed' is the first definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'lock' is the second definition.
(both can mean a piece of hair)
'from' acts as a link.
Can you help me to learn more?
(Other definitions for tress that I've seen before include "Long piece of hair" , "is a bit of a shock" , "Switch" , "Rests (anag.)" , "Obsolete support" .)