Possibly the deep ulterior motive (3)
I believe the answer is:
end
'motive' is the definition.
(thesaurus)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for end that I've seen before include "Closure" , "Unyielding" , "Extreme" , "Remainder" , "Goal; leftover" .)