Obstruction encountered in Little Thurrock (3)
I believe the answer is:
let
'obstruction' is the definition.
(let can mean a hindrance)
'encountered in little thurrock' is the wordplay.
'encountered in' indicates the answer is hidden within the clue.
'LET' can be found hidden inside 'little thurrock'.
(Other definitions for let that I've seen before include "Allow or lease" , "Rent out (property)" , "Allow; leased" , "Service that must be repeated" , "Allowed or rented out" .)