Lieutenant in complete retreat (7)
I believe the answer is:
shelter
'retreat' is the definition.
(shelter can mean a retreat or hideout)
'lieutenant in complete' is the wordplay.
'lieutenant' becomes 'lt' (abbreviation for lieutenant).
'in' means one lot of letters goes inside another.
'complete' becomes 'sheer' (I've seen this in another clue).
'lt' inserted within 'sheer' is 'SHELTER'.
(Other definitions for shelter that I've seen before include "Give protection" , "Sanctuary, protection" , "Place giving protection" , "Roof" , "Refuge; protect" .)