Tricky warning device in tricky case (6)
I believe the answer is:
thorny
'tricky' is the definition.
(thesaurus)
'warning device in tricky case' is the wordplay.
'warning device' becomes 'horn' (horn is a kind of warning device).
'in' means one lot of letters goes inside another.
'case' means to remove the middle letters.
'tricky' with its middle removed is 'ty'.
'horn' going into 'ty' is 'THORNY'.
(Other definitions for thorny that I've seen before include "Problematic; having spines" , "Not easy" , "Prickly, problematic" , "Spiny - difficult and controversial" , "(Of problem) causing difficulty; prickly" .)