Grey Geneva, maybe, lacking a leader? It's a Swiss hallmark (10)
I believe the answer is:
neutrality
'lacking a leader? it's a swiss hallmark' is the definition.
I can't tell whether this definition defines the answer.
'grey geneva' is the wordplay.
'grey' becomes 'neutral' (I've seen this before).
'geneva' becomes 'ity' (I can't justify this - if you can you should believe this answer much more).
'neutral'+'ity'='NEUTRALITY'
'maybe' is the link.
Can you help me to learn more?
(Other definitions for neutrality that I've seen before include "Non-aligned state" , "Impartial policy" , "Detachment" .)