Nut in middle of open tin (5)
I believe the answer is:
pecan
'nut' is the definition.
(pecan is a kind of nut)
'middle of open tin' is the wordplay.
'middle of' indicates the central letters.
'tin' becomes 'can' ('can' can be a synonym of 'tin').
The central letters of 'open' are 'pe'.
'pe'+'can'='PECAN'
'in' acts as a link.
(Other definitions for pecan that I've seen before include "Edible nut like a smooth walnut" , "Species of hickory - edible nut" , "Walnut-like nut" , "Pinkish-brown nut used in American pie" , "Nut with sweet oily kernel" .)