The most common and most likely answer to this crossword clue is the 6 letter word DIPPER
I've seen this crossword clue in The Daily Telegraph
D
I
P
P
E
R
Buddy explains!
The straight part
We know that "Bird/big one seen in sky at night" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case bird/big one seen in sky at night = dipper.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 DIPPER mean?
DIPPER noun- One who, or that which, dips (immerses something, or itself, into a liquid).
- Any of various small passerine birds of the genus Cinclus that live near fast-flowing streams and feed along the bottom.
- A cup-shaped vessel with a long handle, for dipping into and ladling out liquids; a ladle or scoop.
- The control in a vehicle that switches between high-beam and low-beam (i.e. dips the lights), especially when used to signal other vehicles.
- Any snack food intended to be dipped in sauce.
- A pickpocket.
- A person employed in a tin plate works to coat steel plates in molten tin by dipping them.
- A person employed to assist a bather in and out of the sea.
- A Baptist or Dunker.