Notice a large number appear (4)
I believe the answer is:
'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' ().
(Other definitions for seem that I've seen before include "Come across as" , "Appear, look" , "Appear, give impression of being" , "Appear to the eye" .)