Thinks us to be in a bit of a mess! (5)
I believe the answer is:
muses
'thinks' is the definition.
(musing is a kind of thinking)
'us to be in a bit of a mess' is the wordplay.
'to be in' indicates putting letters inside.
'a bit of a mess' becomes 'mes' (I can't explain this - if you can you should believe this answer much more).
'us' inserted into 'mes' is 'MUSES'.
Can you help me to learn more?
(Other definitions for muses that I've seen before include "Ponders like Greek goddesses" , "Inspirational female group" , "Nine goddesses who presided over learning and the arts" , "In Greek and Roman myth, nine goddesses of the arts" , "Nine Greek and Roman goddesses presiding over the arts" .)