In a manner of speaking, concealing a piece of evidence (10)
I believe the answer is:
indication
'piece of evidence' is the definition.
'in a manner of speaking concealing a' is the wordplay.
'a manner of speaking' becomes 'diction' (I've seen this before).
'concealing' means one lot of letters goes inside another.
'diction' going around 'a' is 'dication'.
'in'+'dication'='INDICATION'
(Other definitions for indication that I've seen before include "it shows" , "form of evidence" , "Sign (of something)" .)