Expert breaks into stately mansion at first? That's easy (2,7)
I believe the answer is:
no problem
'that's easy' is the definition.
'expert breaks into stately mansion at first?' is the wordplay.
'expert' becomes 'pro' (short for professional).
'breaks into' means one lot of letters goes inside another.
'stately' becomes 'noble' (synonyms).
'at first?' indicates taking the first letters (I've seen 'at first' mean this).
The first letter of 'mansion' is 'm'.
'pro' put inside 'noble' is 'noproble'.
'noproble'+'m'='NO PROBLEM'
(Other definitions for no problem that I've seen before include "fine" , "Don't worry about it!" , "of course" , "Easy peasy!" .)