Chaps repeatedly separating atomic isotopes of hydrogen, for a change (9)
I believe the answer is:
amendment
'for a change' is the definition.
(amendment is a kind of change)
'chaps repeatedly separating atomic isotopes of hydrogen' is the wordplay.
I cannot really understand how this works, but
'chaps' could be 'men' (chap can mean a man) and 'men' is located in the answer.
'atomic' could be 'a' (eg in A-test, atomic test) and 'a' is found within the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for amendment that I've seen before include "Addition or alteration, to a motion or official document, say" , "the fifth one benefiting Uncle Sam's defendants?" , "Minor improvement" , "Corrective treatment" , "A qualification" .)