Advertise a steak in Tweets regularly (7)
I believe the answer is:
trumpet
'advertise' is the definition.
(to trumpet something is to advertise it)
'a steak in tweets regularly' is the wordplay.
'a steak' becomes 'rump' (rump steak is a kind of steak).
'in' means one lot of letters goes inside another.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternate letters of 'tweets' are 'tet'.
'rump' put within 'tet' is 'TRUMPET'.
(Other definitions for trumpet that I've seen before include "Loud cry" , "Pert tum (anag.)" , "Army instrument for signals" , "Brass musical instrument" , "beastly noise" .)