In fact, the gathering is about due to finish (6)
I believe the answer is:
really
'in fact' is the definition.
(synonyms)
'gathering is about due to finish' is the wordplay.
I cannot really see how this works, but
'about' could be 'rea' (I've seen this in other clues) and 'rea' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
'the' is the link.
Can you help me to learn more?
(Other definitions for really that I've seen before include "Undoubtedly" , "Do you mean it?" , "Indeed" , "Honestly, it's a fact" , "In truth - join forces again" .)