That's right officer, it's a deposit (4)
I believe the answer is:
silt
'a deposit' is the definition.
(I know that silt is a type of deposit)
'that's right officer it's' is the wordplay.
'right' suggests the final letters (the letter on the right of the word).
'officer' becomes 'il' (I can't justify this - if you can you should give a lot more credence to this answer).
'it' becomes ''t' (abbreviation. e.g. in 'tis).
The final letter of 'thats' is 's'.
's'+'il'+'t'='SILT'
Can you help me to learn more?
(Other definitions for silt that I've seen before include "Sedimentary deposit" , "Sediment in a stream" , "Material deposited by a river" , "Sedimentary material" , "Sediment deposited by water as in channel" .)