Many an advert seems crazy! (3)
I believe the answer is:
mad
'crazy' is the definition.
(I know that crazy can be written as mad)
'many an advert' is the wordplay.
'many' becomes 'm' (Roman numeral for a thousand).
'an advert' becomes 'ad' (synonyms).
'm'+'ad'='MAD'
'seems' acts as a link.
(Other definitions for mad that I've seen before include "Insane - furious" , "Crazed" , "lunatic" , "Frenzied" , "like the March Hare" .)