Eastern prince is open in retrospect (4)
I believe the answer is:
raja
'eastern prince' is the definition.
(I've seen this in another clue)
'open in retrospect' is the wordplay.
'open' becomes 'ajar' (similar in meaning).
'in retrospect' says the letters should be written backwards (in retrospect means looking back).
'ajar' in reverse letter order is 'RAJA'.
'is' is the link.
(Other definitions for raja that I've seen before include "Prominent Indian" , "Indian prince or king" , "sovereign" , "Indian ruler" , "Asian prince or king" .)