What a rash twit Thomas is! (9)
I believe the answer is:
foolhardy
'what a rash' is the definition.
(synonyms)
'twit thomas is' is the wordplay.
I cannot really understand how this works, but
'twit' could be 'fool' (I've seen this before) and 'fool' is located in the answer.
The remaining letters 'hardy' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for foolhardy that I've seen before include "Bold" , "daredevil" , "Rash - incautious" , "Impulsive" , "Hot-headed" .)