The interval is expressed in feet (4)
I believe the answer is:
paws
'feet' is the definition.
(I know this)
'the interval is expressed' is the wordplay.
'the interval' becomes 'pause' (pause is a kind of interval).
'is expressed' shows a homophone (sound like).
'pause' is a homophone of 'PAWS'.
'in' acts as a link.
(Other definitions for paws that I've seen before include "Parts of Cats" , "Tom's feet" , "Feet of a quadruped" .)