In a surprise attack, am exhausted (8)
I believe the answer is:
ambushed
'in a surprise attack' is the definition.
I can't judge whether this defines the answer.
'am exhausted' is the wordplay.
'exhausted' becomes 'bushed' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'am'+'bushed'='AMBUSHED'
Can you help me to learn more?
(Other definitions for ambushed that I've seen before include "Attacked (someone) from concealment" , "Lay in wait and waylaid" , "taken, surprisingly" , "Taken by surprise in war" , "surprised?)" .)