Gangster flash northern detective nuts (7)
I believe the answer is:
almonds
'nuts' is the definition.
(almond is a kind of nut)
'gangster flash northern detective' is the wordplay.
'gangster' becomes 'al' (Al Capone).
'flash' becomes 'mo' (both mean a very short period of time).
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'detective' becomes 'DS' (detective sergeant).
'al'+'mo'+'n'+'ds'='ALMONDS'
(Other definitions for almonds that I've seen before include "Nuts used to make marzipan" , "Fruits for marzipan" , "Man sold (anag.)" , "Sold man on kind of nuts" .)