Be enthusiastic about Democrat leading US state transmission (5,4)
I believe the answer is:
radio wave
'transmission' is the definition.
(radio wave is a type of transmission)
'be enthusiastic about democrat leading us state' is the wordplay.
'be enthusiastic' becomes 'rave' (as in rave reviews).
'about' is an insertion indicator.
'leading' suggests taking the first letters.
'us state' becomes 'iowa' (I've seen this before).
The initial letter of 'democrat' is 'd'.
'd'+'iowa'='diowa'
'rave' enclosing 'diowa' is 'RADIO WAVE'.
(Other definitions for radio wave that I've seen before include "Carrier of sound" , "Transmission signal" , "Long-distance message-carrier" .)