Lieutenant wearing flimsy protection (7)
I believe the answer is:
shelter
'protection' is the definition.
(shelter is a kind of protection)
'lieutenant wearing flimsy' is the wordplay.
'lieutenant' becomes 'lt' (abbreviation for lieutenant).
'wearing' indicates putting letters inside.
'flimsy' becomes 'sheer' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'lt' put into 'sheer' is 'SHELTER'.
Can you help me to learn more?
(Other definitions for shelter that I've seen before include "Sanctuary, protection" , "Protection from bad weather or danger" , "Place offering protection" , "Sanctuary; protect" , "Refuge; protect" .)