Flying by the touchline? (2,3,4)
I believe the answer is:
on the wing
'flying' is the definition.
(I've seen this before)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for on the wing that I've seen before include "Where feathers are" , "down the touchline" , "Where one used to see 7 or 11" , "in flight?" , "Like birds, often" .)