Between a couple, the tango is very important (5)
I believe the answer is:
vital
'very important' is the definition.
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for vital that I've seen before include "Essential to life, indispensable" , "Critical, necessary" , "Essential for life; key" , "Spirited" , "Of key importance" .)