Ecstasy, as it happens, will keep our group undefinable (7)
I believe the answer is:
elusive
'undefinable' is the definition.
Both the answer and definition are adjectives. Maybe you can see an association between them that I can't see?
'ecstasy as it happens will keep our group' is the wordplay.
'ecstasy' becomes 'E' (informal term for the drug).
'as it happens' becomes 'live' (as in live television).
'will keep' is an insertion indicator.
'our group' becomes 'us'.
'live' enclosing 'us' is 'lusive'.
'e'+'lusive'='ELUSIVE'
Can you help me to learn more?
(Other definitions for elusive that I've seen before include "Sue about Levi, being shifty" , "Hard to find" , "Hard to grasp" , "Evasive or intangible" , "Hard to pin down or remember" .)