Rely excessively on old and extremely vulnerable scheme (7)
I believe the answer is:
overuse
'rely excessively' is the definition.
I can't judge whether this defines the answer.
'old and extremely vulnerable scheme' is the wordplay.
'old' becomes 'o' (abbreviation as in OT - Old Testament).
'and' is a charade indicator (letters next to each other).
'extremely' means to remove the middle letters (only the extremes of the word are used).
'scheme' becomes 'ruse' (I've seen this in another clue).
'vulnerable' with its middle removed is 've'.
'o'+'ve'+'ruse'='OVERUSE'
'on' is the link.
Can you help me to learn more?
(Other definitions for overuse that I've seen before include "Employ excessively" , "Employ too often" , "Excessive employment" , "Employ too much" , "Wear out" .)