True ally upset about nothing (5)
I believe the answer is:
loyal
'true' is the definition.
(I've seen this before)
'ally upset about nothing' is the wordplay.
'upset' indicates anagramming the letters.
'about' indicates putting letters inside.
'nothing' becomes 'o' (O resembles 0).
'ally' anagrammed gives 'lyal'.
'lyal' enclosing 'o' is 'LOYAL'.
(Other definitions for loyal that I've seen before include "Keeping faith" , "Faithful - true" , "one can trust" , "Dedicated" , "True to one's allegiance" .)