Fool may be unwelcome pain in the backside (5)
I believe the answer is:
goose
'fool' is the definition.
(goose is a kind of fool)
'be unwelcome pain in the backside' is the wordplay.
I cannot really understand how this works, but
'be' could be 'go' (going is a kind of being) and 'go' is found in the answer.
This may be the basis of clue (or it may be nonsense).
'may' acts as a link.
Can you help me to learn more?
(Other definitions for goose that I've seen before include "Grab seat" , "Silly person" , "Migratory aquatic bird" , "Web-footed long-necked bird" , "Pimply state of the skin when cold or scared (with a bird)" .)