Twitter CEO helped its rivals prosper at first (5)
I believe the answer is:
chirp
'twitter' is the definition.
(I know that twitter is a more specific form of the action chirp)
'ceo helped its rivals prosper at first' is the wordplay.
'at first' indicates taking the first letters.
The initial letters of 'ceo helped its rivals prosper' is 'CHIRP'.
(Other definitions for chirp that I've seen before include "Make sound like grasshopper" , "Short sharp tone of grasshopper" , "Sound made by small bird" , "Tweet" , "Fledgling's sound" .)