Nocturnal flyer is close to top container (8)
I believe the answer is:
nightjar
'nocturnal flyer' is the definition.
'nightjar' can be an answer for 'flyer' (nightjar is a type of bird, something which flies). I am not sure about the 'nocturnal' bit.
'close to top container' is the wordplay.
'close' becomes 'nigh' (similar in meaning).
'top' suggests taking the first letters (in a down clue, the first letter is top).
'container' becomes 'jar' (jar is a kind of container).
The initial letter of 'to' is 't'.
'nigh'+'t'+'jar'='NIGHTJAR'
'is' is the link.
Can you help me to learn more?
(Other definitions for nightjar that I've seen before include "Nocturnal bird related to the swift" , "Nocturnal insect-eating bird" , "flier" , "Winger" , "Nocturnal bird of swift family" .)