Plane not working at full speed (4,3)
I believe the answer is:
flat out
'at full speed' is the definition.
(I've seen this before)
'plane not working' is the wordplay.
'plane' becomes 'flat' (synonyms).
'not working' becomes 'out' (out can mean on strike).
'flat'+'out'='FLAT OUT'
(Other definitions for flat out that I've seen before include "Exhausted - at full speed" , "Ready to drop" , "Using every effort" , "At top speed; intense (effort)" , "As fast as possible" .)