Take one flower as told, it's yellow and in a jar (10)
I believe the answer is:
piccalilli
'jar' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for piccalilli that I've seen before include "Spicy pickle" , "Relish with chopped pickled cucumbers, green peppers and onion" , "Spicy relish" , "Chopped vegetables, mustard and hot spices" , "Hot pickle" .)