Reduce car’s energy going round motorway (8)
I believe the answer is:
minimise
'reduce' is the definition.
('reduce'->'minimize' is in my internal thesaurus and 'ize'->'ise')
'car's energy going round motorway' is the wordplay.
'car's' becomes 'minis' (Mini Cooper).
'energy' becomes 'e' (physics symbol as in E=mci).
'going round' is an insertion indicator (some letters go around others).
'motorway' becomes 'MI' (resembles Mm, English motorway).
'minis'+'e'='minise'
'minise' placed around 'mi' is 'MINIMISE'.
(Other definitions for minimise that I've seen before include "Downplay, understate" , "Reduce to the smallest possible amount" , "Shrink" , "Diminish" , "Semi I'm in (anag)" .)