Publication hit back following a merger (7)
I believe the answer is:
amalgam
'merger' is the definition.
'publication hit back following a' is the wordplay.
'publication' becomes 'mag' (abbreviation for magazine).
'hit' becomes 'lam' (to 'lam' means to hit hard).
'back' shows that the letters should be reversed in order.
'following' means one lot of letters go next to another.
'mag'+'lam'='maglam'
'maglam' in reverse letter order is 'malgam'.
'malgam' after 'a' is 'AMALGAM'.
(Other definitions for amalgam that I've seen before include "Mixture (for dental fillings)" , "Mixture/blend" , "Combination of diverse elements" , "Alloy of mercury and another metal" , "combination of things" .)