NCO in mess with a hot snack (5)
I believe the answer is:
nacho
'snack' is the definition.
(I know that nacho is a type of tortilla chip)
'nco in mess with a hot' is the wordplay.
'in mess with' indicates anagramming the letters.
'hot' becomes 'h' (abbreviation eg on taps).
'a'+'h'='ah'
'nco' anagrammed with 'ah' makes 'NACHO'.
(Other definitions for nacho that I've seen before include "Mexican dish" , "kind of chip" , "Mexican tortilla chip" , "Mexican bite" , "Tortilla chip served with savoury toppings" .)