If I play a Pokemon with the "Top Entry" Ability and it goes to the bench as a Stage 1, what happens if I then use Devolution Spray Z?
You can't use Devolution Spray Z on it since although it is an "Evolution Pokemon", it is not Evolved because there is no Basic underneath it. Either you can't play the card, or if it has already been played (it's no longer in the hand) another target will have to be chosen. If there are no other legal targets, then the play has to be reversed.
Source: TPCi Rules Team (2021-01-21)

Relevant cards

Note: All applicable cards may not be listed below.
SWSH2_8
SWSH2_8
SWSH2_32
SWSH2_32
SWSH2_61
SWSH2_61
SM10_166
SM10_166