Courage of London police let out (6)
I believe the answer is:
mettle
'courage' is the definition.
(mettle is a kind of courage)
'london police let out' is the wordplay.
'london police' becomes 'met' (London Metropolitan Police).
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'let' anagrammed gives 'tle'.
'met'+'tle'='METTLE'
'of' acts as a link.
(Other definitions for mettle that I've seen before include "Quality" , "Bravery" , "Strength of character, courage" , "Courage sounds like iron, for instance" , "boldness" .)