Effective new set is most tidy (7)
I believe the answer is:
neatest
'most tidy' is the definition.
('neat' can be similar in meaning to 'tidy')
'effective new set' is the wordplay.
'effective' becomes 'neat' (I can't explain this - if you can you should give a lot more credence to this answer).
'new' is an anagram indicator.
'set' is an anagram of 'est'.
'neat'+'est'='NEATEST'
'is' is the link.
Can you help me to learn more?
(Other definitions for neatest that I've seen before include "best-looking desk, say?" , "Seat ten (anag)" , "Most tidy" , "Most smart" , "Most orderly" .)