Inspector thanks a witness (9)
I believe the answer is:
spectator
'witness' is the definition.
(I know that witness can be written as spectator)
'inspector thanks a' is the wordplay.
I cannot really see how this works, but
'thanks' could be 'ta' (UK informal version of 'thank you') and 'ta' is located in the answer.
The remaining letters 'spector' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for spectator that I've seen before include "member of the crowd" , "Witness" , "observer" , "One attends match" , "one's seen on the terraces?" .)