Stand up, as might pitcher? (4,5)
I believe the answer is:
hold water
'stand up' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for hold water that I've seen before include "make sense?" , "Window-cleaners use buckets to" , "wash" , "Kettle will usually" , "Be credible" .)