Not the one Theo managed to get right! (5)
I believe the answer is:
other
'not the one' is the definition.
(ie another one)
'theo managed to get right' is the wordplay.
'managed' indicates an anagram.
'to get right' becomes 'r' (I've seen this in other clues).
'theo' with letters rearranged gives 'othe'.
'othe'+'r'='OTHER'
(Other definitions for other that I've seen before include "Not the one mentioned" , "One may be significant" , "Alternate; additional" , "Different, alternative" , "Distinct" .)