Royal Navy pilot admits being less lenient (7)
I believe the answer is:
sterner
'less lenient' is the definition.
(I've seen this in another clue)
'royal navy pilot admits' is the wordplay.
'royal navy' becomes 'RN' (abbreviation for Royal Navy).
'pilot' becomes 'steer' (both can mean to direct).
'admits' means one lot of letters goes inside another.
'rn' going within 'steer' is 'STERNER'.
'being' is the link.
(Other definitions for sterner that I've seen before include "Less easy-going" , "More demanding" , "Renters of more strict sort" , "More grimly serious" , "more inflexible anyway" .)