Caused to be insane before end of life (4)
I believe the answer is:
made
'caused' is the definition.
('make' can be a synonym of 'cause')
'insane before end of life' is the wordplay.
'insane' becomes 'mad' ('mad' can be a synonym of 'insane').
'before' says to put letters next to each other.
'end of' indicates one should take the final letters.
The final letter of 'life' is 'e'.
'mad'+'e'='MADE'
'to be' acts as a link.
(Other definitions for made that I've seen before include "Created" , "Constructed" , "Produced" , "Forced" , "brought about" .)