Be careful — yobbo packing punch that takes head off (4,3)
I believe the answer is:
look out
'be careful' is the definition.
(I've seen this before)
'yobbo packing punch that takes head off' is the wordplay.
'yobbo' becomes 'lout' (I've seen this before).
'packing' indicates putting letters inside (some letters are packed inside).
'punch' becomes 'hook' (type of punch in boxing).
'that takes head off' means to remove the first letter (I've seen 'taking head off' mean this).
'hook' with its first letter taken off is 'ook'.
'lout' going around 'ook' is 'LOOK-OUT'.
(Other definitions for look out that I've seen before include "Be on your guard!" , "A view of the future" , "Be watchful" , "Take care" , "as Wenceslas did" .)