Left Isaac, just the same (4)
I believe the answer is:
like
'same' is the definition.
(I know that same can be written as like)
'left isaac just' is the wordplay.
'left' becomes 'L' (common abbreviation).
'isaac just' becomes 'ike' (I can't justify this - if you can you should believe this answer much more).
'l'+'ike'='LIKE'
'the' acts as a link.
Can you help me to learn more?
(Other definitions for like that I've seen before include "Find enjoyable" , "a match?" , "See 27 Across" , "something similar" , "Approve; be fond of" .)