Be uncertain to complete an unfinished new tube (5)
I believe the answer is:
doubt
'be uncertain' is the definition.
(I've seen this in another clue)
'complete an unfinished new tube' is the wordplay.
I cannot quite understand how this works, but
'complete' could be 'do' (doing is a kind of completing) and 'do' is present in the answer.
This explanation may well be incorrect...
'to' acts as a link.
Can you help me to learn more?
(Other definitions for doubt that I've seen before include "Have reservations or be uncertain about" , "Lack of conviction" , "Don't be so sure!" , "Be uncertain about or consider unlikely" , "Feeling of uncertainty about something" .)