One who might walk down the aisle in plain clothes (5,9)
I believe the answer is:
store detective
'one who might walk down the aisle in plain clothes' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'one who might walk down the' is the wordplay.
I cannot quite understand how this works, but
'one' could be 'o' (I've seen this in other clues) and 'o' is present in the answer.
'down' could be 'd' and 'd' is present in the answer.
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Another definition for store detective that I've seen is " high street official".)