Incidentally, see you around Thursday (2,3,3)
I believe the answer is:
by the bye
'incidentally' is the definition.
('by the bye' can be a synonym of 'incidentally')
'see you around thursday' is the wordplay.
'see you' becomes 'byebye' (I've seen this before).
'around' is an insertion indicator.
'thursday' becomes 'th' ('Th' can be a synonym of 'thursday').
'byebye' enclosing 'th' is 'BY THE BYE'.
(Other definitions for by the bye that I've seen before include "In passing" , "Parenthetically" , "Incidentally" .)