Platitude should suit Mr Mills (6)
I believe the answer is:
truism
'platitude' is the definition.
(I've seen this before)
'suit mr mills' is the wordplay.
'mills' indicates anagramming the letters (letters mill or swarm around).
'suit'+'mr'='suitmr'
'suitmr' with letters rearranged gives 'TRUISM'.
'should' acts as a link.
I am not very happy about this link. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for truism that I've seen before include "An obvious statement that says noting new or interesting" , "Hackneyed statement that is obviously correct" , "Bromide" , "An obvious truth" , "Saying that's obviously accurate" .)