Enforce one medium position (6)
I believe the answer is:
impose
'enforce' is the definition.
(I know that enforce can be written as impose)
'one medium position' is the wordplay.
'one' becomes 'I' (Roman numeral).
'medium' becomes 'm' (as in UK clothing sizes).
'position' becomes 'pose' (synonyms).
'i'+'m'+'pose'='IMPOSE'
(Other definitions for impose that I've seen before include "Intrude or inflict upon" , "Decree" , "Put on, tax or obligation" , "Be a bother" , "Force (something) to be done or accepted" .)