Inside measurement is key for collar (5)
I believe the answer is:
seize
'collar' is the definition.
(I know that collar is a more specific form of seize)
'inside measurement is key for' is the wordplay.
'inside' means one lot of letters goes inside another.
'measurement' becomes 'size' (size is a kind of measurement).
'is key for' becomes 'e' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'size' enclosing 'e' is 'SEIZE'.
Can you help me to learn more?
(Other definitions for seize that I've seen before include "Grasp forcibly" , "Grasp hold of" , "Grab by force" , "Nobble" , "Snatch" .)