Old king to tread on chicken (7)
I believe the answer is:
stephen
'old king' is the definition.
(Stephen of Blois, King of England 1135-1154)
'tread on chicken' is the wordplay.
'tread' becomes 'step' ('step' can be a synonym of 'tread').
'on' means one lot of letters go next to another.
'chicken' becomes 'hen' (hen is a kind of chicken).
'step'+'hen'='STEPHEN'
'to' is the link.
(Other definitions for stephen that I've seen before include "-- Hawking, English physicist" , "Boy" , "Male" , "Author of The Shining" , "Man" .)