Period intervening in poor sentence (8)
I believe the answer is:
'period' is the definition.
(meantime is a kind of period)
'poor sentence' is the wordplay.
'poor' becomes 'mean' (similar in meaning).
'sentence' becomes 'time' (a prison sentence).
'intervening in' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for meantime that I've seen before include "interval" , "For the moment" , "Intervening period" , "interim?" .)