Happens old subject is the same, but of different weight (8)
I believe the answer is:
isotopic
'the same but of different weight' is the definition.
I can't judge whether this definition defines the answer.
'happens old subject' is the wordplay.
'happens' becomes 'is' (to be can mean to happen).
'old' becomes 'o' (abbreviation).
'subject' becomes 'topic' (as in a topic for discussion).
'is'+'o'+'topic'='ISOTOPIC'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for isotopic that I've seen before include "Like different forms of the same element" , "like some atoms" , "Relating to different forms of an element" .)