Tool, when turned, let us in (7)
I believe the answer is:
utensil
'tool' is the definition.
(thesaurus)
'turned let us in' is the wordplay.
'turned' indicates anagramming the letters.
'let'+'us'+'in'='letusin'
'letusin' with letters rearranged gives 'UTENSIL'.
'when' is the link.
(Other definitions for utensil that I've seen before include "knife, maybe" , "A container for kitchen use" , "Mandolin, perhaps" , "Implement, maybe in kitchen" , "A kitchen instrument or container" .)