Express disapproval of keeping one in decline (10)
I believe the answer is:
depreciate
'decline' is the definition.
(thesaurus)
'express disapproval of keeping one' is the wordplay.
'express disapproval of' becomes 'deprecate' (I've seen this before).
'keeping' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'deprecate' placed around 'i' is 'DEPRECIATE'.
'in' is the link.
(Other definitions for depreciate that I've seen before include "Belittle; lose value" , "Lose value with time" , "Gradually lose value" .)