Luke in France one had clearly understood (5)
I believe the answer is:
lucid
'clearly understood' is the definition.
(I've seen this in another clue)
'luke in france one had' is the wordplay.
'luke in france' becomes 'luc' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'one had' becomes 'Id' ('I'd' can mean 'one had').
'luc'+'id'='LUCID'
Can you help me to learn more?
(Other definitions for lucid that I've seen before include "Clearly expressed and altogether rational" , "Clear, sane" , "Quite clear" , "Easy to understand, clearly expressed" , "Sensible" .)