Ruin promising start in earth (5)
I believe the answer is:
spoil
'ruin' is the definition.
(I've seen this in another clue)
'promising start in earth' is the wordplay.
'start' says to take the initial letters.
'in' is an insertion indicator.
'earth' becomes 'soil' (earth can mean soil or dirt).
The initial letter of 'promising' is 'p'.
'p' going within 'soil' is 'SPOIL'.
(Other definitions for spoil that I've seen before include "Waste from mining; mar" , "Destroy or ruin" , "Cook" , "Diminish or destroy the value or quality" , "Wreck" .)