Additional matter written about lake (4)
I believe the answer is:
plus
'additional' is the definition.
(I've seen this before)
'matter written about lake' is the wordplay.
'matter' becomes 'pus' (pus is a kind of matter).
'written about' indicates putting letters inside.
'lake' becomes 'l'.
'pus' enclosing 'l' is 'PLUS'.
(Other definitions for plus that I've seen before include "sign of more to come" , "An advantage (colloq.)" , "being a little cross" , "Distant" , "Positive quality" .)