Provide a fake alibi for, say (4)
I believe the answer is:
abet
'provide a fake alibi for say' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for abet that I've seen before include "Beat up by way of help" , "Beat (anag.)" , "Assist wrongly!" , "Aid and help, perhaps in crime" , "Assist (a criminal)" .)