Occasion prisoner causes delay (4,3)
I believe the answer is:
time lag
'delay' is the definition.
(a time lag is a delay)
'occasion prisoner' is the wordplay.
'occasion' becomes 'time' (I've seen this in another clue).
'prisoner' becomes 'lag' (slang term).
'time'+'lag'='TIME-LAG'
'causes' is the link.
(Other definitions for time lag that I've seen before include "Gap between linked events" , "Delay" , "interval" .)