Agreed to put all the world on a new diet (6)
I believe the answer is:
united
'agreed' is the definition.
(similar in meaning)
'all the world on a new diet' is the wordplay.
'all the world' becomes 'un' (I can't justify this - if you can you should give a lot more credence to this answer).
'on' indicates putting letters inside.
'a new' indicates an anagram.
'diet' anagrammed gives 'ited'.
'un' inserted within 'ited' is 'UNITED'.
'to put' is the link.
Can you help me to learn more?
(Other definitions for united that I've seen before include "Allied or combined" , "Football team" , "... we stand, divided we fall" , "Manchester _____, current Premiership champions with..." , "Agreed" .)