Start to take blame for ambush? (4)
I believe the answer is:
trap
'ambush?' is the definition.
('trap' can be a synonym of 'ambush')
'start to take blame' is the wordplay.
'start to' says to take the initial letters.
'blame' becomes 'rap' ('rap' can be a synonym of 'blame').
The initial letter of 'take' is 't'.
't'+'rap'='TRAP'
'for' acts as a link.
(Other definitions for trap that I've seen before include "Hook" , "Baited device" , "that can let an actor down" , "Bunker; carriage" , "website?" .)