Dignified admission that one’s putting on an act? (8)
I believe the answer is:
imposing
'dignified admission' is the definition.
'imposing' can be an answer for 'dignified' (thesaurus). I'm not certain of the 'admission' bit.
'one's putting on an act?' is the wordplay.
'one's' becomes 'im' ('one is' is a formal way of saying 'I am').
'putting on' says to put letters next to each other.
'an act?' becomes 'posing' (posing is a kind of act).
'im'+'posing'='IMPOSING'
'that' is the link.
(Other definitions for imposing that I've seen before include "Impressive in appearance" , "Grand, impressive" , "Taking liberties" , "Impressive - or putting tax on" , "Dictating; impressive" .)