I said, left , you see, with one open day easy to understand (5)
I believe the answer is:
lucid
'easy to understand' is the definition.
(I've seen this before)
'i said left you see with one open day' is the wordplay.
'i said left you see' becomes 'luci' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'with' is a charade indicator (letters next to each other).
'one open' suggests taking the first letters (take the opening letter).
The initial letter of 'day' is 'd'.
'luci'+'d'='LUCID'
Can you help me to learn more?
(Other definitions for lucid that I've seen before include "Sensible" , "Clear, easy to understand" , "Clear, easily understood" , "Clear, easily understandable" , "Easy to understand, clearly expressed" .)