Plant after day is OK (12)
I believe the answer is:
satisfactory
'ok' is the definition.
(I've seen this before)
'plant after day is' is the wordplay.
'plant' becomes 'factory' (factory is a kind of plant**).
'after' says to put letters next to each other.
'day' becomes 'sat' (I've seen this before).
'sat'+'is'='satis'
'factory' put after 'satis' is 'SATISFACTORY'.
(Other definitions for satisfactory that I've seen before include "fine" , "OK" , "Fully acceptable" , "unexceptional" , "Up to scratch" .)