Protection of rank protecting lieutenant (7)
I believe the answer is:
shelter
'protection' is the definition.
(I know that protection can be written as shelter)
'rank protecting lieutenant' is the wordplay.
'rank' becomes 'sheer' (as in sheer hypocrisy, rank hypocrisy etc).
'protecting' means one lot of letters goes inside another (inserted letters are held or protected).
'lieutenant' becomes 'lt' (abbreviation for lieutenant).
'sheer' going around 'lt' is 'SHELTER'.
'of' acts as a link.
(Other definitions for shelter that I've seen before include "Refuge, protection" , "Refuge; protect" , "Housing or haven" , "Roof" , "Shield as from rain" .)