See what's in thus? (6)
I believe the answer is:
behold
'see' is the definition.
(I know that behold is a type of see)
'what's in thus?' is the wordplay.
'what' becomes 'eh' (sound used to ask 'what?' or 'pardon?').
'in' is an insertion indicator.
'thus?' becomes 'bold' (I can't justify this - if you can you should believe this answer much more).
'eh' put inside 'bold' is 'BEHOLD'.
Can you help me to learn more?
(Other definitions for behold that I've seen before include "Archaic injunction to look" , "Look upon" , "Look at with attention" , "Lo, see here" , "Lo - look!" .)