Stop a satellite beginning to decay (6)
I believe the answer is:
period
'stop' is the definition.
(I know that stop can be written as period)
'a satellite beginning to decay' is the wordplay.
'a' becomes 'per' (eg 'one a day' means 'one per day').
'satellite' becomes 'Io' (Io is a moon of Jupiter).
'beginning to' indicates taking the first letters.
The initial letter of 'decay' is 'd'.
'per'+'io'+'d'='PERIOD'
(Other definitions for period that I've seen before include "Stretch - cycle" , "End of story" , "Stop in America" , "Duration or interval" , "Length of time, or a full stop" .)