The majority are in Half Moon Street (4)
I believe the answer is:
most
'the majority' is the definition.
(most of a group)
'half moon street' is the wordplay.
'half' indicates taking half.
'street' becomes 'st' (abbreviation).
'moon' with half the letters taken is 'mo'.
'mo'+'st'='MOST'
'are in' acts as a link.
(Other definitions for most that I've seen before include "In the highest degree" , "Nearly all, majority" , "Almost all" , "Greatest in amount or degree" , "Extremely, very" .)