Lieutenant protected by very thin shield (7)
I believe the answer is:
shelter
'shield' is the definition.
(I've seen this before)
'lieutenant protected by very thin' is the wordplay.
'lieutenant' becomes 'lt' (abbreviation for lieutenant).
'protected by' indicates putting letters inside (inserted letters are held or protected).
'very thin' becomes 'sheer' (I've seen this in another clue).
'lt' inserted into 'sheer' is 'SHELTER'.
(Other definitions for shelter that I've seen before include "Refuge, protection" , "Protection from bad weather or danger" , "Covering that provides protection from the weather" , "Housing or haven" , "retreat" .)