Language of terrible boaster carrying zero credit (5-5)
I believe the answer is:
serbo-croat
'language' is the definition.
(I've seen this before)
'terrible boaster carrying zero credit' is the wordplay.
'terrible' indicates anagramming the letters.
'carrying' indicates putting letters inside.
'zero' becomes 'O' (O resembles 0).
'credit' becomes 'cr' (abbreviation).
'boaster' with letters rearranged gives 'serboat'.
'o'+'cr'='ocr'
'serboat' placed around 'ocr' is 'SERBO-CROAT'.
'of' acts as a link.
(Other definitions for serbo-croat that I've seen before include "European" , "language" .)