Conclusion not yet decided, head of panel having left (6)
I believe the answer is:
ending
'left' is the definition.
The definition and answer can be both acts as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'conclusion not yet decided head of panel' is the wordplay.
'conclusion' becomes 'end' (end is a kind of conclusion).
'not yet decided head of panel' becomes 'ing' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'end'+'ing'='ENDING'
'having' acts as a link.
(Other definitions for ending that I've seen before include "Armageddon?" , "Climax" , "Last part" , "Bringing to a close" , "Gin den (anag.)" .)