Officer covered by very thin protection (7)
I believe the answer is:
shelter
'protection' is the definition.
(shelter is a kind of protection)
'officer covered by very thin' is the wordplay.
'officer' becomes 'lt' (I've seen this in other clues).
'covered by' is an insertion indicator.
'very thin' becomes 'sheer' (I've seen this before).
'lt' going into 'sheer' is 'SHELTER'.
(Other definitions for shelter that I've seen before include "Roof" , "Place offering protection" , "retreat" , "Protection; charity for the homeless" , "Monk's hood" .)