Measure old court, when leaving (4)
I believe the answer is:
size
'measure' is the definition.
(size is a kind of measure)
'old court when leaving' is the wordplay.
'old court' becomes 'assize' (I've seen this before).
'when' becomes 'as'.
'leaving' suggests deleting specific letters.
'assize' with 'as' taken out is 'SIZE'.
(Other definitions for size that I've seen before include "Volume" , "mass" , "Dimension" , "Standard measurement" , "gelatinous mixture" .)