Good enough outside, it's a fantastic small plant (12)
I believe the answer is:
satisfactory
'good enough' is the definition.
(I've seen this before)
'outside it's a fantastic small plant' is the wordplay.
'outside' means one lot of letters goes inside another.
'fantastic' indicates an anagram.
'small' becomes 's' (abbreviation - e.g. clothes size).
'plant' becomes 'factory' (factory is a kind of plant**).
'its'+'a'='itsa'
'itsa' is an anagram of 'atis'.
's'+'factory'='sfactory'
'atis' inserted within 'sfactory' is 'SATISFACTORY'.
(Other definitions for satisfactory that I've seen before include "Good enough" , "unexceptional" , "Meeting expectations" , "Up to scratch" , "Acceptable - adequate" .)