Rhythm is just keeping time (5)
I believe the answer is:
'rhythm' is the definition.
(I've seen this before)
'just keeping time' is the wordplay.
'just' becomes 'mere' (I've seen this in other clues).
'keeping' means one lot of letters goes inside another.
'time' becomes 't'.
'mere' going around 't' is 'METRE'.
'is' acts as a link.
(Other definitions for metre that I've seen before include "Moisture remover" , "measure of length" , "Beat pattern (mus.)" , "1.094 yards" , "39.37 inches" .)