Enforce one medium posture (6)

I believe the answer is:

impose

'enforce' is the definition.
(I know that enforce can be written as impose)

'one medium posture' is the wordplay.
'one' becomes 'i' (Roman numeral).
'medium' becomes 'm' (as in UK clothing sizes).
'posture' becomes 'pose' (synonyms).
'i'+'m'+'pose'='IMPOSE'

(Other definitions for impose that I've seen before include "Decree" , "As task, set" , "Obtrude, force (on)" , "Put burden or tax on" , "Force upon or intrude" .)

I've seen this clue in The Mirror.

Want a hint initially instead of a full solution? Install my app