Endorse it for a short period (6)
I believe the answer is:
second
'endorse' is the definition.
(I know that endorse can be written as second)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for second that I've seen before include "Support, assist" , "Flash" , "Back (but not first!)" , "Moment; back" , "Short time - back" .)