Leave desk after arranging to go off (9)
I believe the answer is:
skedaddle
'leave' is the definition.
(I know that skedaddle is a more specific form of the action break away)
'desk after arranging to go off' is the wordplay.
'after arranging' indicates an anagram.
'to go off' becomes 'addle' (eg an egg might addle).
'desk' with letters rearranged gives 'sked'.
'sked'+'addle'='SKEDADDLE'
(Other definitions for skedaddle that I've seen before include "bolt" , "Scurry off" , "run away" , "split" , "Vamoose" .)