Lieutenant in rank providing cover (7)
I believe the answer is:
shelter
'cover' is the definition.
(thesaurus)
'lieutenant in rank' is the wordplay.
'lieutenant' becomes 'lt'.
'in' is an insertion indicator.
'rank' becomes 'sheer' ('sheer' can be a synonym of 'rank'**).
'lt' placed within 'sheer' is 'SHELTER'.
'providing' acts as a link.
(Other definitions for shelter that I've seen before include "'Refuge, cover (7)'" , "Sanctuary; protect" , "Housing or haven" , "harbour" , "Place of refuge" .)