Head case turns around clue seen many times before (8)
I believe the answer is:
chestnut
'times before' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'head case turns around clue seen many' is the wordplay.
I cannot really see how this works, but
'head' could be 'nut' (informal term for somebody's head) and 'nut' is located in the answer.
'case' could be 'chest' (both are types of container) and 'chest' is found in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for chestnut that I've seen before include "''Underneath the spreading . . . . . . . . tree''" , "Sweet -; horse -; water -" , "Old joke - edible kernel" , "Stale joke or cliche" , "Old story" .)