Mass commercial for crackers (3)
I believe the answer is:
mad
'crackers' is the definition.
(crackers can informally mean mad)
'mass commercial' is the wordplay.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'commercial' becomes 'ad' (abbreviation for advertisement).
'm'+'ad'='MAD'
'for' acts as a link.
(Other definitions for mad that I've seen before include "Berserk" , "Idiotic" , "Frantic" , "Insane - furious" , "lunatic" .)