Start down — you moved left with your back to the wall (2,7)

'with your back to the wall' is the definition.
(someone backed against the wall is in trouble)

'start down you moved left' is the wordplay.
I cannot quite see how this works, but
'start' could be 'intro' (introduction) and 'intro' is found within the answer.
'you' could be 'u' (commonly used in online communication) and 'u' is found within the answer.
'left' could be 'l' (common abbreviation) and 'l' is found in the answer.
The remaining letters 'be' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).