Almost complete? Not quite! Small line is missing (5)
I believe the answer is:
about
'almost' is the definition.
(I've seen this before)
'complete? not quite small line is missing' is the wordplay.
'complete? not quite small line' becomes 'ab' (I can't explain this - if you can you should give a lot more credence to this answer).
'is missing' becomes 'out' (I've seen this in another clue).
'ab'+'out'='ABOUT'
Can you help me to learn more?
(Other definitions for about that I've seen before include "not far away" , "Regarding" , "Approximately, or in vicinity" , "in the offing?" , "Concerning; approximately" .)