Surprise as some Birmingham bus hijacked (6)
I believe the answer is:
ambush
'surprise' is the definition.
(a surprise attack)
'some birmingham bus hijacked' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'AMBUSH' is hidden within 'birmingham bus hijacked'.
'as' is the link.
(Other definitions for ambush that I've seen before include "Wait in hiding to attack" , "Lying in wait to attack" , "Launch attack from hiding" , "(Where enemies) lie in wait" , "Waylay" .)