Fix one medium factory (7)
I believe the answer is:
implant
'fix' is the definition.
(thesaurus)
'one medium factory' is the wordplay.
'one' becomes 'i' (Roman numeral).
'medium' becomes 'm' (as in UK clothing sizes).
'factory' becomes 'plant' (a manufacturing plant).
'i'+'m'+'plant'='IMPLANT'
(Other definitions for implant that I've seen before include "New element in trap" , "Fix securely and deeply" , "Establish" , "Graft into the body" , "material (medical)" .)