Polish knight has gone blue (3)
I believe the answer is:
sad
'blue' is the definition.
(similar in meaning)
'polish knight has gone' is the wordplay.
'polish' becomes 'sand' (to polish or sand down).
'knight' becomes 'N' (chess abbreviation).
'has gone' suggests deleting specific letters.
'sand' with 'n' removed is 'SAD'.
(Other definitions for sad that I've seen before include "Melancholy, depressed" , "Unfortunate" , "Dismal" , "low" , "Mournful and depressed" .)