Some cause or purpose (3)
I believe the answer is:
use
'purpose' is the definition.
('use' can be a synonym of 'purpose')
'some cause or' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'USE' is hidden within 'cause or'.
(Other definitions for use that I've seen before include "Exploit unfairly" , "Avail oneself of" , "Benefit" , "Custom - service" , "Value" .)