Puck's old hat? Deadlock ensues ... (7)
I believe the answer is:
impasse
'deadlock ensues' is the definition.
I can't tell whether this defines the answer.
'puck's old hat?' is the wordplay.
'puck's' becomes 'im' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'old hat?' becomes 'passe' (I've seen this before).
'im'+'passe'='IMPASSE'
Can you help me to learn more?
(Other definitions for impasse that I've seen before include "block" , "Maybe gridlock" , "Lack of movement" , "Stalling of negotiations" , "Inescapable perdicament, dilemma" .)