The old fool jeers stupidly about a shirt tail (6)
I believe the answer is:
jester
'the old fool' is the definition.
'jester' can be an answer for 'fool' (synonyms). I'm not certain of the 'the old' bit.
'jeers stupidly about a shirt tail' is the wordplay.
'stupidly' is an anagram indicator (a stupid ordering of the letters).
'about' is an insertion indicator.
'tail' says to take the final letters.
The final letter of 'shirt' is 't'.
'jeers' is an anagram of 'jeser'.
'jeser' going around 't' is 'JESTER'.
Can you help me to learn more?
(Other definitions for jester that I've seen before include "Court entertainer (with bells on?)" , "Court fool" , "Court clown" , "Yorick" , "Professional clown of the middle ages" .)