Announce a line that must be crossed by crowd (6)
I believe the answer is:
herald
'announce' is the definition.
(synonyms)
'a line that must be crossed by crowd' is the wordplay.
'line' becomes 'l'.
'that must be crossed by' indicates putting letters inside.
'crowd' becomes 'herd' (synonyms).
'a'+'l'='al'
'al' inserted within 'herd' is 'HERALD'.
(Other definitions for herald that I've seen before include "Official crier or messenger" , "Proclaimer" , "College of Arms official" , "Proclaim the approach of, like archaic messenger" , "Person who announces important news" .)