Hampers one medium speed, strangely (7)
I believe the answer is:
impedes
'hampers' is the definition.
(I've seen this before)
'one medium speed strangely' is the wordplay.
'one' becomes 'i' (Roman numeral).
'medium' becomes 'm' (as in UK clothing sizes).
'strangely' indicates anagramming the letters.
'speed' with letters rearranged gives 'pedes'.
'i'+'m'+'pedes'='IMPEDES'
(Other definitions for impedes that I've seen before include "Hampers" , "Obstructs, blocks" , "Hinders, obstructs" , "checks" .)