Expert turned up in case of poison nut (5)
I believe the answer is:
pecan
'nut' is the definition.
(pecan is a kind of nut)
'expert turned up in case of poison' is the wordplay.
'expert' becomes 'ace' (ace is a kind of expert**).
'turned up' says the letters should be written backwards.
'in' indicates putting letters inside.
'case of' means to remove the middle letters (outsides of).
'poison' with its middle taken out is 'pn'.
'ace' reversed gives 'eca'.
'eca' inserted into 'pn' is 'PECAN'.
(Other definitions for pecan that I've seen before include "Something like a walnut" , "Nutty type" , "of nuts" , "Nut with sweet oily kernel" , "nut-tree" .)