They go round endlessly creating a bit of a mystery (5)
I believe the answer is:
tyres
'a bit of a mystery' is the definition.
I can't tell whether this defines the answer.
'they go round endlessly creating' is the wordplay.
'they go' becomes 'tres' (I can't justify this - if you can you should believe this answer much more).
'round' is an insertion indicator.
'endlessly' means to look at the middle letters (the ends of the words are removed).
'creating' becomes 'copying' (copying is a kind of creating).
The central letter of 'copying' is 'y'.
'tres' going around 'y' is 'TYRES'.
Can you help me to learn more?
(Other definitions for tyres that I've seen before include "Wheel coverings" , "Rubber wheel covers" , "Wheel hoops" , "rubber rings" .)