Many a sad ending is nonsensical (3)
I believe the answer is:
mad
'nonsensical' is the definition.
(thesaurus)
'many a sad ending' is the wordplay.
'many' becomes 'm' (**).
'ending' suggests the final letters.
The final letter of 'sad' is 'd'.
'm'+'a'+'d'='MAD'
'is' acts as a link.
(Other definitions for mad that I've seen before include "Deranged" , "Crazed" , "Aberrant" , "Extremely foolish" , "Roused to anger" .)