Lethargy fiancee Ruth is occasionally beginning to address (7)
I believe the answer is:
inertia
'lethargy' is the definition.
(both can mean a lack of movement)
'fiancee ruth is occasionally beginning to address' is the wordplay.
'occasionally' means one should take alternating letters (letters taken at regular intervals or occasions).
'beginning to' says to take the initial letters.
The first letter of 'address' is 'a'.
'fiancee'+'ruth'+'is'='fianceeruthis'
The alternate letters of 'fianceeruthis' are 'inerti'.
'inerti'+'a'='INERTIA'
(Other definitions for inertia that I've seen before include "Torpor" , "Unwillingness to move" , "Chemical or other inactivity" , "A disposition to remain inactive" , "Listlessness" .)