Deposit is taken back by lieutenant (4)
I believe the answer is:
silt
'deposit' is the definition.
(I know that silt is a type of deposit)
'is taken back by lieutenant' is the wordplay.
'taken back' says the letters should be written in reverse.
'by' means one lot of letters go next to another.
'lieutenant' becomes 'lt' (abbreviation for lieutenant).
'is' written backwards gives 'si'.
'si'+'lt'='SILT'
(Other definitions for silt that I've seen before include "Sediment deposited in channel or harbour" , "Mud deposits in a river" , "Fine deposited material" , "Jamboree" , "Muddy deposit" .)