Criterion followed by a cavalry regiment (8)
I believe the answer is:
standard
'criterion' is the definition.
(I know that criterion can be written as standard)
'followed by a cavalry regiment' is the wordplay.
I cannot really understand how this works, but
'a' could be 'an' and 'an' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for standard that I've seen before include "Normal quality" , "'Reference point, touchstone (8)'" , "Against which to measure the rest" , "Reference point against which other things can be evaluated" , "Accepted model - flag" .)