Look out for live sport, say (6)
I believe the answer is:
beware
'look out for' is the definition.
(I've seen this in another clue)
'live sport say' is the wordplay.
'live' becomes 'be' (synonyms).
'sport' becomes 'wear' (sport can mean to wear or display on one's person).
'say' indicates a 'sounds like' (homophone) clue.
'wear' is a homophone of 'ware'.
'be'+'ware'='BEWARE'
(Other definitions for beware that I've seen before include "Be cautious about" , "Word of warning" , "Archaic word for look out, mind yourself" , "See 18" , "Archaic warning to watch out, danger ahead" .)