Imagine sovereign having no commitments
I believe the answer is:
fancy free
'having no commitments' is the definition.
(I've seen this before)
'imagine sovereign' is the wordplay.
'imagine' becomes 'fancy' (fancying is a kind of imagining).
'sovereign' becomes 'free' ('free' can be similar in meaning to 'sovereign').
'fancy'+'free'='FANCY-FREE'
(Other definitions for fancy free that I've seen before include "Not emotionally involved" , "Not in love, open to new romance" , "Lacking responsibility - not currently in love" , "Not in love or engaged" , "Not committed romantically" .)