Expert thus securing a flat (7)
I believe the answer is:
prosaic
'flat' is the definition.
(I've seen this before)
'expert thus securing a' is the wordplay.
'expert' becomes 'pro' (short for professional).
'thus' becomes 'sic' ('thus' in Latin).
'securing' indicates putting letters inside.
'sic' placed around 'a' is 'saic'.
'pro'+'saic'='PROSAIC'
(Other definitions for prosaic that I've seen before include "Commonplace, unromantic" , "So Capri! (anag)" , "Workaday" , "flat" , "Poetry would not be so" .)