Up to bringing an assembled unit to a successful conclusion (5)
I believe the answer is:
until
'up to' is the definition.
(I've seen this before)
'an assembled unit to a successful conclusion' is the wordplay.
'an assembled' indicates anagramming the letters.
'to' says to put letters next to each other.
'conclusion' says to take the final letters.
The last letter of 'successful' is 'l'.
'unit' with letters rearranged gives 'unti'.
'unti'+'l'='UNTIL'
'bringing' is the link.
(Other definitions for until that I've seen before include "Pending such time as" , "As late as" , "Unlit (anag.)" , "Up to (a point in time)" , "Not beyond (hour)" .)