He's active between rounds for a short time (6)
I believe the answer is:
second
'short time' is the definition.
(second is a kind of short time)
'he's active between rounds' is the wordplay.
I cannot quite see how this works, but
'active' could be 'on' (I've seen this in other clues) and 'on' is found in the answer.
This explanation may well be incorrect...
'for a' is the link.
Can you help me to learn more?
(Other definitions for second that I've seen before include "Support, assist" , "another?" , "Mo" , "Flash" , "Basic unit of time" .)