All upset about Oscar meeting a sticky end, it's true (5)
I believe the answer is:
loyal
'true' is the definition.
(I've seen this before)
'all upset about oscar meeting a sticky end' is the wordplay.
'upset' indicates an anagram.
'about' is an insertion indicator.
'oscar' becomes 'o' (phonetic alphabet: alpha, bravo, charlie etc.).
'meeting' says to put letters next to each other.
'end' says to take the final letters.
The last letter of 'sticky' is 'y'.
'all' is an anagram of 'lal'.
'o'+'y'='oy'
'lal' going around 'oy' is 'LOYAL'.
'it's' acts as a link.
(Other definitions for loyal that I've seen before include "Showing firm allegiance" , "Steadfast and true" , "Alloy (anag.)" , "Faithful (in allegiance)" , "Steadfast in allegiance" .)