Usual standards shown by Royal Marines in small numbers (5)
I believe the answer is:
norms
'usual standards' is the definition.
'norms' can be an answer for 'standards' (norm is a kind of standard). I'm not sure about the 'usual' bit.
'royal marines in small numbers' is the wordplay.
'royal marines' becomes 'RM' (abbreviation).
'in' indicates putting letters inside.
'small numbers' becomes 'nos' ('no' is an abbreviation for number; 'small' indicates a short form).
'rm' going inside 'nos' is 'NORMS'.
'shown by' acts as a link.
Can you help me to learn more?
(Other definitions for norms that I've seen before include "Standards of customary behaviour" , "Authorative standards" , "Customary behaviour, the usual things" , "Rules" , "Standards, averages" .)