Men on the moon were fine once, a member of staff's gathering (9)
I believe the answer is:
americans
'men on the moon were' is the definition.
(only American astronauts have walked on the Moon)
'fine once a member of staff's gathering' is the wordplay.
'fine once' becomes 'eric' (historical type of fine paid by a murderer).
'member of staff's' becomes 'man's' (man can mean a worker or member of a staff).
'gathering' indicates putting letters inside.
'a'+'mans'='amans'
'eric' put within 'amans' is 'AMERICANS'.
(Other definitions for americans that I've seen before include "Yanks" , "Rains came (anag.)" , "New Yorkers, e.g." , "across the pond?" .)