Condense source in dictionary, one even now incomplete (6)
I believe the answer is:
distil
'condense' is the definition.
(I've seen this before)
'source in dictionary one even now incomplete' is the wordplay.
'source in' indicates taking the first letters.
'one' becomes 'i' (Roman numeral).
'even now' becomes 'still' (I've seen this in another clue).
'incomplete' means to remove the last letter.
The first letter of 'dictionary' is 'd'.
'still' with its last letter taken away is 'stil'.
'd'+'i'+'stil'='DISTIL'
(Other definitions for distil that I've seen before include "Purify by boiling and condensing" , "Make (spirits)" , "Refine, extract" , "Make, eg, whisky" , "Make more concise" .)