Maybe decrease in tension, originally brought about (4)
I believe the answer is:
iron
'maybe decrease' is the definition.
'iron' can be an answer for 'decrease' (de-crease: remove creases by ironing). I'm not certain of the 'maybe' bit.
'in tension originally brought about' is the wordplay.
'in' says the answer is hidden in the clue.
'brought about' shows that the letters should be reversed in order.
'nori' is hidden within 'tension originally'.
'nori' written backwards gives 'IRON'.
Can you help me to learn more?
(Other definitions for iron that I've seen before include "Smooth; element" , "Press clothes" , "inflexible > club" , "Smooth things" , "Heavy metallic element" .)