Sit outside one with equilibrium (5)
I believe the answer is:
poise
'equilibrium' is the definition.
(poise is a kind of equilibrium)
'sit outside one' is the wordplay.
'sit' becomes 'pose' (synonyms).
'outside' is an insertion indicator.
'one' becomes 'I' (Roman numeral).
'pose' going around 'i' is 'POISE'.
'with' acts as a link.
(Other definitions for poise that I've seen before include "Graceful and elegant bearing in a person" , "Dignity and assurance of manner" , "Sang-froid" , "Elegance" , "Self-confidence" .)