Knight is recalled by king (3)
I believe the answer is:
sir
'knight' is the definition.
(I've seen this before)
'is recalled by king' is the wordplay.
'recalled' says the letters should be written backwards.
'by' says to put letters next to each other.
'king' becomes 'r' (r is abbreviation for rex).
'is' back-to-front is 'si'.
'si'+'r'='SIR'
(Other definitions for sir that I've seen before include "Knight's title" , "Male title of respect" , "Polite form of address to a man" , "as Winston Churchill was" , "Title of 1 across" .)