Lieutenant in utter retreat (7)
I believe the answer is:
shelter
'retreat' is the definition.
(I've seen this before)
'lieutenant in utter' is the wordplay.
'lieutenant' becomes 'lt' (abbreviation for lieutenant).
'in' means one lot of letters goes inside another.
'utter' becomes 'sheer' (synonyms).
'lt' going into 'sheer' is 'SHELTER'.
(Other definitions for shelter that I've seen before include "'Refuge, cover (7)'" , "Sanctuary, protection" , "Monk's hood" , "Protective structure" , "harbour" .)