Time in simple measure (5)
I believe the answer is:
metre
'measure' is the definition.
(metre is a kind of measure)
'time in simple' is the wordplay.
'time' becomes 't'.
'in' is an insertion indicator.
'simple' becomes 'mere' ('mere' can be a synonym of 'simple').
't' inserted within 'mere' is 'METRE'.
(Other definitions for metre that I've seen before include "Yard is smaller than this" , "Approximately 39 inches" , "Basic unit of length" , "measure of length" , "three and a bit of a fourth" .)