Flying down the touchline? (2,3,4)
I believe the answer is:
on the wing
I believe this clue is a double definition.
'flying' is the first definition.
(I've seen this before)
'down the touchline?' is the second definition.
(eg on the wing in football)
(Other definitions for on the wing that I've seen before include "Where one used to see 7 or 11" , "(Of birds) flying" , "in flight?" , "Like birds, often" , "Where feathers are" .)