Mile added into a little circuit (5)
I believe the answer is:
ambit
'circuit' is the definition.
(I've seen this in another clue)
'mile added into a little' is the wordplay.
'mile' becomes 'm' (common abbreviation).
'added into' indicates putting letters inside.
'a little' becomes 'a bit' (synonyms).
'm' inserted within 'abit' is 'AMBIT'.
(Other definitions for ambit that I've seen before include "round" , "Circumference" , "Range, scope" , "Scope or confines" , "Scope, extent, or bounds of something" .)