Different speed on foot (7)
I believe the answer is:
spondee
'foot' is the definition.
(type of metrical foot in poetry)
'different speed on' is the wordplay.
'different' indicates anagramming the letters.
'speed'+'on'='speedon'
'speedon' with letters rearranged gives 'SPONDEE'.
(Other definitions for spondee that I've seen before include "Metric foot of two long syllables" , "No speed required for poetic metrical foot" , "bit of verse" .)