Harmony is essential to lounge suite's style (5)
I believe the answer is:
unity
'harmony' is the definition.
(I've seen this before)
I don't understand how the rest of the clue works.
(Other definitions for unity that I've seen before include "State of being joined as a whole" , "Completeness (with nothing wanting)" , "Accord" , "Solidarity" , "State of togetherness" .)