Girl reflected on porridge served in interval (4,3)
I believe the answer is:
time lag
'interval' is the definition.
(time lag is a kind of interval)
'girl reflected on porridge' is the wordplay.
'girl' becomes 'gal' (synonyms).
'reflected' is a reversal indicator.
'on' is a charade indicator (letters next to each other).
'porridge' becomes 'time' (porridge is slang for time in prison).
'gal' backwards is 'lag'.
'lag' put after 'time' is 'TIME-LAG'.
'served in' acts as a link.
(Other definitions for time lag that I've seen before include "interval" , "Gap between linked events" , "Delay" .)