Man or mouse showing standard (4)
I believe the answer is:
norm
'standard' is the definition.
(I know that norm is a type of standard)
'man or mouse showing' is the wordplay.
'showing' indicates a hidden word (the answer is shown in the clue).
'NORM' can be found hidden inside 'man or mouse'.
(Other definitions for norm that I've seen before include "Standard level of performance" , "Rule; average" , "Usual or standard thing" , "Average level of performance" , "Drawing stick" .)