A warning given previously (5)
I believe the answer is:
afore
'previously' is the definition.
(I've seen this in another clue)
'a warning given' is the wordplay.
'warning' becomes 'fore' (used to warn of airborne balls on a golf course).
'given' means one lot of letters go next to another.
'a'+'fore'='AFORE'
(Other definitions for afore that I've seen before include "in the van a while back" , "formerly" , "In front of" , "Earlier" , "Previously" .)