Sensible king swaps sides first, I see (9)
I believe the answer is:
realistic
'sensible' is the definition.
(I've seen this in another clue)
'first i see' is the wordplay.
'first' becomes 'realist' (I can't justify this - if you can you should believe this answer much more).
'see' becomes 'C' (the word for the letter, according to Chambers).
'realist'+'i'+'c'='REALISTIC'
'king swaps sides' acts as a link.
This may not be right. Some or all of it may be part of another bit of the clue.
(Other definitions for realistic that I've seen before include "accurate picture" , "It is clear, being practical and sensible" , "Practicable" , "believable" , "Practical; true to life" .)