Left one medium object as restriction (5)
I believe the answer is:
limit
'restriction' is the definition.
(I've seen this in another clue)
'left one medium object' is the wordplay.
'left' becomes 'l' (common abbreviation).
'one' becomes 'i' (Roman numeral).
'medium' becomes 'm' (as in UK clothing sizes).
'object' becomes 'it' (pronoun for an inanimate object).
'l'+'i'+'m'+'it'='LIMIT'
'as' is the link.
(Other definitions for limit that I've seen before include "Hem in" , "Put restriction on" , "end" , "Furthest bound" , "Restrict - term" .)