Bag search regularly reveals it (3)
I believe the answer is:
sac
'bag' is the definition.
(I've seen this before)
'search regularly reveals it' is the wordplay.
'regularly reveals it' means one should take alternating letters (letters taken at regular intervals).
The alternating letters of 'search' are 'SAC'.
(Other definitions for sac that I've seen before include "Bag-like cavity" , "Bag-like part of the body" , "Pouch or bag-like structure" , "Membranous cavity" , "Distended membrane; pouch-like cavity" .)