Act as lookout, say (4)
I believe the answer is:
abet
'act as lookout say' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for abet that I've seen before include "Help a criminal" , "Assist (in doing wrong?)" , "Support, aid" , "Aid (a criminal)" , "Assist criminally" .)