Vehicle reversed by man wearing gem, not of the highest standard (10)
I believe the answer is:
suboptimal
'not of the highest standard' is the definition.
'vehicle reversed by man wearing gem' is the wordplay.
'vehicle' becomes 'bus' (I've seen this before).
'reversed' shows that the letters should be reversed in order.
'by man' becomes 'tim' (I am not sure about the 'by' bit.**).
'wearing' indicates putting letters inside.
'gem' becomes 'opal' (opal is a kind of gem).
'bus' reversed gives 'sub'.
'tim' placed into 'opal' is 'optimal'.
'sub'+'optimal'='SUBOPTIMAL'
(Another definition for suboptimal that I've seen is " under par".)