House-sitter keeps applications (4)
I believe the answer is:
uses
'applications' is the definition.
(I've seen this before)
'house-sitter keeps' is the wordplay.
'keeps' indicates the answer is hidden within the clue.
'USES' is hidden within 'house sitter'.
(Other definitions for uses that I've seen before include "Employs" , "Functions, applications" , "Takes (drugs)" , "Exercises" , "Operates" .)