Guard started crossing line (6)
I believe the answer is:
shield
'guard' is the definition.
(thesaurus)
'started crossing line' is the wordplay.
'started' becomes 'shied' (shying is a kind of starting).
'crossing' indicates putting letters inside.
'line' becomes 'l'.
'shied' placed around 'l' is 'SHIELD'.
(Other definitions for shield that I've seen before include "Sporting trophy; protect" , "Is held (anag.)" , "Piece of plated armour" , "Piece of protective armour" , "where you may find your bearings" .)