Unit applied to horse measurement (5)
I believe the answer is:
depth
'measurement' is the definition.
(depth is a kind of measurement)
'unit applied to horse' is the wordplay.
'unit' becomes 'dept' (I can't explain this - if you can you should give a lot more credence to this answer).
'applied to' means one lot of letters go next to another.
'horse' becomes 'H' (abbreviation as in horse power).
'dept'+'h'='DEPTH'
Can you help me to learn more?
(Other definitions for depth that I've seen before include "Downward extent" , "Distance downwards" , "Deepness" , "Measurement from front to back" , "Distance from top down as in water" .)