Appear to understand a lot (4)
I believe the answer is:
seem
'appear' is the definition.
(I know that appear can be written as seem)
'understand a lot' is the wordplay.
'understand' becomes 'see' (synonyms).
'a lot' becomes 'm'.
'see'+'m'='SEEM'
'to' acts as a link.
(Other definitions for seem that I've seen before include "Appear, give impression of being" , "Come across as" , "Appear to the eye" , "Appear, look" .)