Notice a large number appear (4)
I believe the answer is:
seem
'appear' is the definition.
(I know that appear can be written as seem)
'notice a large number' is the wordplay.
'notice' becomes 'see' (notice can mean to spot or see).
'a large number' becomes 'm' ().
'see'+'m'='SEEM'
(Other definitions for seem that I've seen before include "Come across as" , "Appear, look" , "Appear, give impression of being" , "Appear to the eye" .)