What you'd expect Polly to put on to restrict those in March (6)
I believe the answer is:
kettle
'march' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for kettle that I've seen before include "Contain demonstrators" , "Kitchen utensil and type of drum" , "Piece of kitchen equipment" , "Kitchen boiler?" , "Domestic appliance that can whistle?" .)