Longshanks in some measure unleashed war dogs (6)
I believe the answer is:
edward
'longshanks' is the definition.
(nickname of King Edward I)
'some measure unleashed war dogs' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'EDWARD' can be found hidden inside 'measure unleashed war dogs'.
'in' is the link.
(Other definitions for edward that I've seen before include "First name of Mr Rochester (Jane Eyre)" , "The Black Prince" , "The Queen's youngest son" , "Boy" , "Prince born in 1964" .)