Changes nowadays past improving (6)
I believe the answer is:
adapts
'changes' is the definition.
(adapting is a kind of changing)
'nowadays past improving' is the wordplay.
'nowadays' becomes 'AD' (AD, Anno Domini, as opposed to BC, Before Christ).
'improving' is an anagram indicator (letters must be sorted out or rectified).
'past' with letters rearranged gives 'apts'.
'ad'+'apts'='ADAPTS'
(Other definitions for adapts that I've seen before include "Changes to suit new circumstances" , "Modifies or changes for another use" , "Converts to another use, say novel to stage" , "Makes fit for a new purpose" , "Alters for another use" .)