Crossword Genius

Officer lenient but a criminal scratches bishop (10)

Ross

I believe the answer is:

lieutenant

I'm a little stuck... Click here to teach me more about this clue!

'officer' is the definition.
(I know that lieutenant is a type of commissioned naval officer)

'lenient but a criminal scratches bishop' is the wordplay.
I cannot really see how this works, but
'a' could be 'an' and 'an' is found within the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for lieutenant that I've seen before include "UK rank below captain" , "Military rank" , "Army rank" , "e.g. one on French side who had a woman?" , "Army officer" .)

I've seen this clue in The Telegraph.
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