Tip from Dominic Raab: dismissing answer is dull (4)
I believe the answer is:
drab
'is dull' is the definition.
(synonyms)
'tip from dominic raab dismissing answer' is the wordplay.
'tip from' indicates taking the first letters.
'dismissing answer' becomes 'ab' (I can't justify this - if you can you should believe this answer much more).
The first letters of 'dominic raab' is 'dr'.
'dr'+'ab'='DRAB'
Can you help me to learn more?
(Other definitions for drab that I've seen before include "Cheerless, colourless" , "Of a dull greenish-brown colour" , "Not cheering" , "Dreary and colourless" , "Dull light brown colour" .)