Grass on another thief, for starters (3)
I believe the answer is:
oat
'grass' is the definition.
(I've seen this in other clues)
'on another thief for starters' is the wordplay.
'for starters' indicates taking the first letters.
The initial letters of 'on another thief' is 'OAT'.
(Other definitions for oat that I've seen before include "Basis of porridge" , "Cereal grass" , "Food grain" , "Cereal (singular)" , "Cereal plant" .)