Once again work a tiny length and restart (6)
I believe the answer is:
reopen
'restart' is the definition.
Although both the answer and definition are verbs in their base form, I can't see how they can define each other.
'once again work a tiny length and' is the wordplay.
'once again' becomes 're' (prefix in verbs).
'work' becomes 'op' (abbreviation for opus).
'a tiny length' becomes 'e' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'and' becomes 'n' (common abbreviation for 'and').
're'+'op'+'e'+'n'='REOPEN'
Can you help me to learn more?
(Other definitions for reopen that I've seen before include "Begin again" , "Undo again" , "Unlock door - again" , "Unlock or admit customers again" , "Resume investigation or unlock again" .)