Do one's utmost (6)
I believe the answer is:
strive
'do one's utmost' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for strive that I've seen before include "Contend using rivets" , "Endeavour" , "Try hard, carry on conflict" , "Work hard - compete" , "Try hard to break rivets" .)