Rattle heard in part of cycle? (5)
I believe the answer is:
phase
'part of cycle?' is the definition.
(I know that phase is part of cycle)
'rattle heard' is the wordplay.
'rattle' becomes 'faze' (both can mean to bother or unsettle).
'heard' indicates a 'sounds like' (homophone) clue.
'faze' is a homophone of 'PHASE'.
'in' acts as a link.
(Other definitions for phase that I've seen before include "Period or stage in a process of change" , "Passing stage" , "Period within process" , "Distinct stage" , "stage in process of development" .)