Initially Manchester United retains single stand arena (6)
I believe the answer is:
milieu
'arena' is the definition.
(thesaurus)
'initially manchester united retains single stand' is the wordplay.
'initially' suggests taking the first letters.
'retains' indicates putting letters inside.
'single' becomes 'I' (one).
'stand' becomes 'lie' (I can't explain this - if you can you should believe this answer much more).
The initial letters of 'manchester united' is 'mu'.
'i'+'lie'='ilie'
'mu' placed around 'ilie' is 'MILIEU'.
Can you help me to learn more?
(Other definitions for milieu that I've seen before include "Social environment or set of surroundings" , "Environment - setting" , "'Surroundings, setting (6)'" , "(Social) setting" , "Surroundings - location" .)