Only keeping time on some simple rhythms (6)
I believe the answer is:
metres
'rhythms' is the definition.
(metre is the rhythm of a poem or piece of music)
'only keeping time on some simple' is the wordplay.
'only' becomes 'mere' (I've seen this before).
'keeping' is an insertion indicator.
'time' becomes 't' (abbreviation).
'on' means one lot of letters go next to another.
'some' says to take the initial letters.
The initial letter of 'simple' is 's'.
'mere' enclosing 't' is 'metre'.
'metre'+'s'='METRES'
(Other definitions for metres that I've seen before include "Lengths of approximately 1.094 yards" , "measures" , "Units of length" .)