Agree and a small double, neat, is poured out (6)
I believe the answer is:
assent
'agree' is the definition.
(assenting is a kind of agreeing)
'a small double neat is poured out' is the wordplay.
'a small' becomes 's' (abbreviation - e.g. clothes size).
'double' shows that letters should be duplicated.
'is poured' is an anagram indicator (letters get poured or mixed around).
'out' indicates putting letters inside.
's' doubled is 'ss'.
'neat' with letters rearranged gives 'aent'.
'ss' going inside 'aent' is 'ASSENT'.
'and' is the link.
(Other definitions for assent that I've seen before include "Agree - concede" , "Official sanction" , "Give the thumbs-up" , "Say yes" , "Agreement (to a proposal)" .)