Confirm clip is of leading rider (7)
I believe the answer is:
proviso
'rider' is the definition.
(I've seen this before)
'confirm clip is of leading' is the wordplay.
'confirm' becomes 'prove' (proving is a kind of confirming).
'clip' means to remove the last letter (I've seen 'clipped' mean this (the word is shortened or clipped)).
'is of leading' becomes 'iso' (I am not sure about this - if you are sure you should believe this answer much more).
'prove' with its final letter taken away is 'prov'.
'prov'+'iso'='PROVISO'
Can you help me to learn more?
(Other definitions for proviso that I've seen before include "Condition of stipulation of an agreement" , "Condition for VIP or so" , "Rider" , "Clause in contract" , "Something insisted upon as part of an agreement" .)