It's said to be dismissive (5)
I believe the answer is:
scram
'it's said to be dismissive' is the definition.
I can't judge whether this definition defines the answer.
'it's said to be' is the wordplay.
'it's said' becomes 'scr' (I can't justify this - if you can you should believe this answer much more).
'to be' becomes 'am'.
'scr'+'am'='SCRAM'
Can you help me to learn more?
(Other definitions for scram that I've seen before include "Leave quickly" , "Clear off!" , "make emergency shut-down" , "Skedaddle!" , "Vamoose" .)