Determined effort (9)
I believe the answer is:
'determined effort' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for endeavour that I've seen before include "Strive - Inspector Morse's first name" , "The first handle inspector has" , "Aspire; attempt" , "Strive - over a nude (anag)" , "Try to achieve something" .)