Laugh at the German plan that's incomplete (6)
I believe the answer is:
deride
'laugh at' is the definition.
(I've seen this before)
'the german plan that's incomplete' is the wordplay.
'the german' becomes 'der' ('the' in German).
'plan' becomes 'idea' (both can mean an intention).
'that's incomplete' means to remove the last letter.
'idea' with its final letter taken off is 'ide'.
'der'+'ide'='DERIDE'
(Other definitions for deride that I've seen before include "Knock" , "Mock and jeer" , "'Mock, sneer at (6)'" , "Test performance" , "Ridicule, laugh at" .)