Insert minute design in it (7)
I believe the answer is:
implant
'insert' is the definition.
(implanting is a kind of inserting)
'minute design in it' is the wordplay.
'minute' becomes 'm' (abbreviation).
'design' becomes 'plan' ('plan' can be a synonym of 'design'**).
'in' is an insertion indicator.
'm'+'plan'='mplan'
'mplan' inserted inside 'it' is 'IMPLANT'.
(Other definitions for implant that I've seen before include "Artificial insert in body" , "get fixed" , "Insert or fix into the body" , "New element in trap" , "Insert within a body" .)