Think I'm trespassing in someone's grounds (8)
I believe the answer is:
estimate
'think' is the definition.
(thesaurus)
'i'm trespassing in someone's grounds' is the wordplay.
I cannot really understand how this works, but
'i'm' could be 'i' (I've seen this in other clues) and 'i' is found within the answer.
'someone' could be 'mate' (mate is a kind of someone) and 'mate' is found within the answer.
The remaining letters 'est' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for estimate that I've seen before include "Judge to be probable" , "approximate price" , "Reckon, judge" , "Believe" , "Tease Tim (anag.)" .)