One flying down the touchline? (6)
I believe the answer is:
winger
'one flying down the touchline?' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'one flying down the touchline?' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'flying' becomes 'awing' (rare adverb, sometimes spelled a-wing).
'down' is a deletion indicator ('down' can mean missing).
'the touchline?' becomes 'er' (I am not sure about this - if you are sure you should believe this answer much more).
'awing' with 'a' taken away is 'wing'.
'wing'+'er'='WINGER'
Can you help me to learn more?
(Other definitions for winger that I've seen before include "Bird" , "on the football field?" , "Footballer" , "attacking sportsman" , "Attacking soccer player" .)