Association’s nicest day out (9)
I believe the answer is:
syndicate
'association's' is the definition.
(syndicate is a kind of association)
'nicest day out' is the wordplay.
'out' indicates an anagram (out can mean wrong or inaccurate).
'nicest'+'day'='nicestday'
'nicestday' anagrammed gives 'SYNDICATE'.
(Other definitions for syndicate that I've seen before include "We might enter lottery" , "Association of people working together on a single project." , "Consortium" , "Organised group of individuals" , "Group of promoters with a common interest" .)