Standard choice of sides for Mancunian (4)
I believe the answer is:
norm
'standard choice' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
(Other definitions for norm that I've seen before include "Standard, average" , "Rule, standard, pattern" , "Bacterium, virus, etc" , "'Standard, usual (4)'" , "What is conventional, regular" .)