Nearly American on 'Fifty-Second Street' (6)
I believe the answer is:
almost
'nearly' is the definition.
('almost' can be a synonym of 'nearly')
'american on fifty-second street' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'on' says to put letters next to each other.
'fifty' becomes 'l' ('L' can be a synonym of 'fifty').
'second' becomes 'mo' (both are short periods of time**).
'street' becomes 'st'.
'a'+'l'+'mo'+'st'='ALMOST'
(Other definitions for almost that I've seen before include "As good as" , "Not entirely" , "Well-nigh" , "Virtually" , "practically" .)