Hide mostly hidden (6)
I believe the answer is:
secret
'hidden' is the definition.
(I've seen this before)
'hide mostly' is the wordplay.
'hide' becomes 'secrete' (I've seen this before).
'mostly' means to remove the last letter.
'secrete' with its last letter taken away is 'SECRET'.
(Other definitions for secret that I've seen before include "Security classification level" , "away from scrutiny" , "Cloak and dagger" , "Security marking" , "Not to be disclosed" .)