Swimmer's aid -- something to help one see coming up round Orkney -- not half (7)
I believe the answer is:
snorkel
'swimmer's aid something to help one' is the definition.
I can't judge whether this definition defines the answer.
'coming up round orkney not half' is the wordplay.
'coming' becomes 'lens' (I can't explain this - if you can you should believe this answer much more).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'round' is an insertion indicator.
'not half' means to take half the letters.
'lens' in reverse letter order is 'snel'.
'orkney' with half the letters taken is 'ork'.
'snel' enclosing 'ork' is 'SNORKEL'.
'see' acts as a link.
Can you help me to learn more?
(Other definitions for snorkel that I've seen before include "Breathing aid for a swimmer" , "Tube supplying air to swimmer" , "Diver's breathing tube" , "Swimmer's air tube" , "Bent tube for a swimmer's mouth" .)