Lease covers to become sick, retired and dependent (7)
I believe the answer is:
reliant
'dependent' is the definition.
(thesaurus)
'lease covers to become sick retired' is the wordplay.
'lease' becomes 'rent' ('rent' can be a synonym of 'lease').
'covers' is an insertion indicator.
'to become sick' becomes 'ail' (I've seen this before).
'retired' is a reversal indicator.
'ail' in reverse letter order is 'lia'.
'rent' going around 'lia' is 'RELIANT'.
'and' is the link.
(Other definitions for reliant that I've seen before include "Banking" , "Dependent (on someone or something)" , "car firm" , "Trusting" , "needing help from others" .)