Thomas, we messed up a bit (8)
I believe the answer is:
somewhat
'a bit' is the definition.
('somewhat' means 'a bit' or 'slightly')
'thomas we messed up' is the wordplay.
'messed up' is an anagram indicator.
'thomas'+'we'='thomaswe'
'thomaswe' is an anagram of 'SOMEWHAT'.
(Other definitions for somewhat that I've seen before include "up to a point" , "Rather, a little" , "Moderately" , "To a certain degree - a bit" , "Quite" .)