Complete idiot ultimately has messed up a lot (5)
I believe the answer is:
total
'complete' is the definition.
(similar in meaning)
'idiot ultimately has messed up a lot' is the wordplay.
'ultimately' indicates one should take the final letters.
'has' means one lot of letters go next to another.
'messed up' indicates an anagram.
The last letter of 'idiot' is 't'.
'a'+'lot'='alot'
'alot' anagrammed gives 'otal'.
't'+'otal'='TOTAL'
(Other definitions for total that I've seen before include "Aggregate amount" , "Complete, absolute" , "Damage beyond repair" , "to give the final figure" , "Comprehensive" .)