This was a British deception designed to conceal the objective of 'Manna' (i) from the Greeks (September 1944).
This was a nugatory undertaking whose creation and implementation was forced on the unwilling Brigadier Dudley W. Clarke’s 'A' Force by General Sir Henry Maitland Wilson, the Supreme Allied Commander Mediterranean, in the autumn of 1944. Wilson demanded that a deception plan, designed to cover the British preparations for the occupations of Athens, the Greek capital, be crafted to disguise the preparations as presaging amphibious assaults on the Istrian peninsula at the head of the Adriatic Sea and on Rhodes in the Dodecanese islands group.
The nugatory element of 'Undercut' was that it was already clear that the German forces in Greece were concerned merely with their evacuation back into Yugoslavia and not with attempting to prevent any British landing or stemming any subsequent advance. Thus there was no strategic, operational or tactical need for 'Undercut', but nonetheless demanded that it proceed in order to keep his plans a secret in Greece itself, where the political situation was complex and volatile.
Clarke pointed out that he had no machinery for deceiving Greeks and that an attempt to operate a deception plan under such circumstances would be pointless and even, perhaps, detrimental to the Allied cause. Clarke was right: the Greek forces involved in 'Manna' (i) protested at the thought that they were about to be committed to combat somewhere other than Greece, and the fact that Greek resistance leaders were brought to Italy to meet the commander of the expedition made the whole thing absurd.
So 'Undercut' was cancelled, and later there was a more apposite 'Second Undercut'.