Grow desperate protecting knight errant (5)
I believe the answer is:
wrong
'errant' is the definition.
(thesaurus)
'grow desperate protecting knight' is the wordplay.
'desperate' indicates anagramming the letters (a desperate or pathetic spelling).
'protecting' means one lot of letters goes inside another (inserted letters are held or protected).
'knight' becomes 'n' (chess abbreviation).
'grow' anagrammed gives 'wrog'.
'wrog' going around 'n' is 'WRONG'.
(Other definitions for wrong that I've seen before include "Treat unjustly" , "Injury - incorrect" , "Unjust act" , "Incorrect - sinful" , "Incorrect, illegal or immoral" .)