It was appropriate at one time to assemble (4)
I believe the answer is:
meet
This might be a double definition.
'it was appropriate at one time' is the first definition.
The definition suggests a singular noun which matches the answer.
'assemble' is the second definition.
(I know that meet is a is a more specific form of the action assemble)
'to' is the link.
Can you help me to learn more?
(Other definitions for meet that I've seen before include "Hunting gathering" , "Sporting encounter" , "Come together or run into" , "It is proper to encounter" , "Come face to face - it is fitting" .)