Clegg's first to be taken in by new leader's broadcast (7)
I believe the answer is:
declare
'broadcast' is the definition.
(thesaurus)
'clegg's first to be taken in by new leader's' is the wordplay.
'first' says to take the initial letters.
'to be taken in by' indicates putting letters inside.
'new' indicates an anagram.
The first letter of 'clegg' is 'c'.
'leader' anagrammed gives 'delare'.
'c' put within 'delare' is 'DECLARE'.
(Other definitions for declare that I've seen before include "Make a statement, to customs perhaps" , "State clearly, announce officially" , "Assert" , "... cut short innings" , "Proclaim" .)