Fifteen new changes involving old chemical company that's wasteful (11)
I believe the answer is:
inefficient
'wasteful' is the definition.
(similar in meaning)
'fifteen new changes involving old chemical company' is the wordplay.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'changes' is an anagram indicator.
'involving' is an insertion indicator.
'old chemical company' becomes 'ICI' (former company Imperial Chemical Industries).
'fifteen'+'n'='fifteenn'
'fifteenn' is an anagram of 'ineffent'.
'ineffent' going around 'ici' is 'INEFFICIENT'.
'that's' acts as a link.
(Other definitions for inefficient that I've seen before include "Not achieving maximum productivity" , "Failing to make the best use of time or resources" , "Achieving low productivity" , "lacking competence" .)