Muppet who once testified before Congress (4)
I believe the answer is:
elmo
'muppet who once testified before congress' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for elmo that I've seen before include "Puppet character" , "Saint associated with fire" , "Saint whose fire is electric!" , "Fiery saint" , "St ...'s fire, weather phenomenon of coloured glow" .)