Being rash has to bring the ultimate penalty (5)
I believe the answer is:
hasty
'being rash' is the definition.
(I've seen this before)
'has to bring the ultimate penalty' is the wordplay.
'to bring' becomes 't' (I can't explain this - if you can you should give a lot more credence to this answer).
'the ultimate' says to take the final letters.
The final letter of 'penalty' is 'y'.
'has'+'t'+'y'='HASTY'
Can you help me to learn more?
(Other definitions for hasty that I've seen before include "Reckless; hurried" , "Impetuous - quick-tempered" , "Rashly quick" , "Hurried; without thinking" , "Rash; thoughtless" .)