Certainly concealing new desires (4)
I believe the answer is:
yens
'desires' is the definition.
(yen is a kind of desire)
'certainly concealing new' is the wordplay.
'certainly' becomes 'yes' (both can be used to answer in the affirmative).
'concealing' is an insertion indicator.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'yes' enclosing 'n' is 'YENS'.
(Other definitions for yens that I've seen before include "Hankerings, yearnings" , "Desires" .)