Would such a cap suit jolly Roger? (5)
I believe the answer is:
skull
'jolly roger?' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for skull that I've seen before include "Bony skeleton of the head" , "Bony frame of head ..." , "The head-bone" , "Bony part of head" , "brains here" .)