The most common and most likely answer to this crossword clue is the 6 letter word GUTTER
I've seen this crossword clue in The Daily Telegraph
G
U
T
T
E
R
More clues leading to the result GUTTER
- One filleting fish maybe, good and thorough
- Channel crossing's final part, say
- Channel close to Viking, say
- Almost go out, heading for garden, say
- Threaten to go out to speak after close of meeting
- Finally reaching, say, the lowest point to reach?
- Threaten to go out on t’pull when returning
- About time! Pull up at the side of the road
- Key needed with complete part of drainage system
- Channel showing ‘Midnight Express’
Buddy explains!

The straight part
We know that "part of drainage system" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case part of drainage system = gutter.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does GUTTER mean?
GUTTER noun- A prepared channel in a surface, especially at the side of a road adjacent to a curb, intended for the drainage of water.
- A ditch along the side of a road.
- A duct or channel beneath the eaves of a building to carry rain water; eavestrough.
- A groove down the sides of a bowling lane.
- A large groove (commonly behind animals) in a barn used for the collection and removal of animal excrement.
- Any narrow channel or groove, such as one formed by erosion in the vent of a gun from repeated firing.
- A space between printed columns of text.
- One of a number of pieces of wood or metal, grooved in the centre, used to separate the pages of type in a form.
- An unprinted space between rows of stamps.
- A drainage channel.
- The notional locus of things, acts, or events which are distasteful, ill bred or morally questionable.
- A low, vulgar state.
- The spaces between comic book panels
- To flow or stream; to form gutters.
- (of a candle) To melt away by having the molten wax run down along the side of the candle.
- (of a small flame) To flicker as if about to be extinguished.
- To send (a bowling ball) into the gutter, not hitting any pins.
- To supply with a gutter or gutters.
- To cut or form into small longitudinal hollows; to channel.
- One who or that which guts.