An average period for an interval (8)
I believe the answer is:
meantime
'for an interval' is the definition.
(meantime is a kind of interval)
'an average period' is the wordplay.
'an average' becomes 'mean' (type of average).
'period' becomes 'time period' (period of time).
'mean'+'time'='MEANTIME'
(Other definitions for meantime that I've seen before include "interval" , "interim?" , "For the moment" , "Intervening period" .)