Parents are lost in the case of rickets (5)
I believe the answer is:
rears
'parents' is the definition.
('rear' can be a synonym of 'parent')
'are lost in the case of rickets' is the wordplay.
'lost' indicates an anagram.
'in the' indicates putting letters inside.
'case of' suggests removing the centre (outsides of).
'rickets' with its centre taken out is 'rs'.
'are' anagrammed gives 'ear'.
'ear' inserted inside 'rs' is 'REARS'.
(Other definitions for rears that I've seen before include "Back ends" , "Sets up" , "Raises" , "bucks" , "Fosters, brings up" .)