Joint holder's fast time running round a school (8)
I believe the answer is:
ligament
'joint' is the definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe there's an association between them I don't understand?
'holder's fast time running round a school' is the wordplay.
'holder's fast' becomes 'lien' (I am not sure about this - if you are sure you should believe this answer much more).
'time' becomes 't' (abbreviation).
'running round' is an insertion indicator.
'a school' becomes 'gam' (I've seen this in other clues).
'lien'+'t'='lient'
'lient' enclosing 'gam' is 'LIGAMENT'.
Can you help me to learn more?
(Other definitions for ligament that I've seen before include "Tissue joining bones or cartilages" , "Tough band in joint" , "What may get torn" , "Joint connector" , "Link between bones or cartilages" .)