This definition is also found at Zonaras delta p.573.18, and (with συνθείς ) at Photius delta 744, cf. Hsch. delta 2311. The metaphor of putting together a dramatic performance for "staging" or manipulating political situations is a favorite of Anna Comnena (e.g. Alexias 1.12.9.7) and others of her time. The word also implies wicked acts, and the overtone is almost certainly to be sought in the use by historians for political manipulation.
The word in the definition is an aorist participle active (as is the headword), from συν-ίημι . It alternates in the lexica with the rather similar form of the same participle from συν-τίθημι . Students of Greek remember the difficulty identifying these two forms.