They form attachments with some guests about to start lunch (5)
I believe the answer is:
glues
'they form attachments' is the definition.
The definition suggests an adverb but the answer is not.
'some guests about to start lunch' is the wordplay.
'some guests' becomes 'gues' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'about' means one lot of letters goes inside another.
'to start' suggests taking the first letters.
The initial letter of 'lunch' is 'l'.
'gues' placed around 'l' is 'GLUES'.
'with' acts as a link.
Can you help me to learn more?
(Another definition for glues that I've seen is " Sticks together".)