Seed planted in the deep earth (3)
I believe the answer is:
pea
'seed' is the definition.
(I've seen this before)
'planted in the deep earth' is the wordplay.
'planted in' says the answer is hidden in the clue.
'PEA' is hidden in the letters of 'deep earth'.
(Other definitions for pea that I've seen before include "registered by princess in bed" , "The plant is a strange ape" , "Vegetable in a whistle" , "It grows in a pod" , "Common vegetable seed from pod" .)