Mix a tub and a quarter in: that’s more than enough (8)
I believe the answer is:
abundant
'that's more than enough' is the definition.
(I've seen this before)
'mix a tub and a quarter' is the wordplay.
'mix' indicates an anagram.
'a quarter' becomes 'n'.
'a'+'tub'+'and'+'n'='atubandn'
'atubandn' with letters rearranged gives 'abundant'.
'in' is the link.
(Other definitions for abundant that I've seen before include "'Copious, galore (8)'" , "Bountiful" , "Present in large quantities" , "Copious, profuse" , "Luxuriant" .)