Worried, I got nervier at questioning (13)
I believe the answer is:
interrogative
'questioning' is the definition.
(I've seen this before)
'worried i got nervier at' is the wordplay.
'worried' indicates anagramming the letters.
'i'+'got'+'nervier'+'at'='igotnervierat'
'igotnervierat' anagrammed gives 'INTERROGATIVE'.
(Other definitions for interrogative that I've seen before include "Denoting a question" , "raising questions" , "Questioning" .)