Polite knight coming in to buy fish (7)
I believe the answer is:
genteel
'polite' is the definition.
(I know that polite can be written as genteel)
'knight coming in to buy fish' is the wordplay.
'knight' becomes 'n' (chess abbreviation).
'coming in to' means one lot of letters goes inside another.
'buy' becomes 'get' (both can mean to purchase or obtain).
'fish' becomes 'eel' (eel is a type of fish).
'n' placed within 'get' is 'gent'.
'gent'+'eel'='GENTEEL'
(Other definitions for genteel that I've seen before include "Polite and well-bred" , "Marked by refinement in taste and manners" , "Courteous, well-bred" , "Affectedly polite and refined" , "Overly polite or refined" .)