One of the sides at Old Trafford? (9)
I believe the answer is:
touchline
'one' is the definition.
Although both the answer and definition are singular nouns, I don't understand how one could define the other.
'the sides at old trafford?' is the wordplay.
I cannot quite understand how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found within the answer.
'at' could be 'in' (synonymous in some cases - eg at school, in school) and 'in' is present in the answer.
'old' could be 'o' (abbreviation) and 'o' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'of' is the link.
(Other definitions for touchline that I've seen before include "side of pitch" , "Rugby/soccer pitch marking" .)