Take advantage of one medium position (6)
I believe the answer is:
impose
'take advantage' is the definition.
(I've seen this in another clue)
'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'
'of' acts as a link.
(Other definitions for impose that I've seen before include "Make demands of" , "Be a bother" , "As task, set" , "Levy or force upon" , "Foist" .)