That's under the bottom of soap dish (3)
I believe the answer is:
pie
'dish' is the definition.
(I've seen this before)
'that's under the bottom of soap' is the wordplay.
'that's' becomes 'ie'.
'under' is a charade indicator (letters next to each other).
'the bottom of' says to take the final letters.
The last letter of 'soap' is 'p'.
'ie' after 'p' is 'PIE'.
(Other definitions for pie that I've seen before include "Bird - dish - mixed type" , "A baked dish covered with pastry" , "Slapstick missile" , "something with custard, say" , "Dish baked in pastry-lined pan" .)