Cast pie out though it's germ-free (7)
I believe the answer is:
aseptic
'it's germ-free' is the definition.
(I've seen this before)
'cast pie out' is the wordplay.
'out' is an anagram indicator (out can mean wrong or inaccurate).
'cast'+'pie'='castpie'
'castpie' anagrammed gives 'ASEPTIC'.
'though' acts as a link.
(Other definitions for aseptic that I've seen before include "Free from bacteria, surgically clean" , "ideal conditions for hospital?" , "Free from bacteria, surically clean" , "Paciest (anag.)" , "Free of bacterial or viral contamination" .)