Big Serb I fancy beginning to hate nonsensical talk (9)
I believe the answer is:
gibberish
'nonsensical talk' is the definition.
(I've seen this before)
'big serb i fancy beginning to hate' is the wordplay.
'fancy' indicates anagramming the letters.
'beginning to' says to take the initial letters.
The initial letter of 'hate' is 'h'.
'big'+'serb'+'i'='bigserbi'
'bigserbi' with letters rearranged gives 'gibberis'.
'gibberis'+'h'='GIBBERISH'
(Other definitions for gibberish that I've seen before include "Nonsensical speech" , "Nonsensical talk" , "moonshine" , "rhubarb" , "Gobbledygook" .)