Fix motorway design inside it (7)
I believe the answer is:
implant
'fix' is the definition.
(thesaurus)
'motorway design inside it' is the wordplay.
'motorway' becomes 'M' (as in motorway names - Mp, Mn etc).
'design' becomes 'plan' (synonyms).
'inside' means one lot of letters goes inside another.
'm'+'plan'='mplan'
'mplan' put within 'it' is 'IMPLANT'.
(Other definitions for implant that I've seen before include "Introduce" , "Something surgically grafted into body" , "Fix firmly" , "Insert in tissue" , "New element in trap" .)