Make quite clear it's a cause of tension (6)
I believe the answer is:
stress
'make quite clear' is the definition.
(to stress something is to be very clear about it)
'it's a cause of tension' is the wordplay.
I cannot really understand how this works, but
'it's' could be 'es' ('e' can mean 'electronic' which is similar to 'IT') and 'es' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for stress that I've seen before include "Lay emphasis on - it's a strain" , "Ictus" , "Underline - tension" , "Feeling of pressure" , "possible cause of fracture?" .)