Unbroken horse that's brother to one in the army (6)
I believe the answer is:
bronco
'unbroken horse' is the definition.
'bronco' can be an answer for 'horse' (bronco is a kind of horse). I'm not certain of the 'unbroken' bit.
'brother to one in the army' is the wordplay.
'brother' becomes 'bro' (short for brother).
'to one in the army' becomes 'nco' (I can't explain this - if you can you should give a lot more credence to this answer).
'bro'+'nco'='BRONCO'
'that's' is the link.
Can you help me to learn more?
(Other definitions for bronco that I've seen before include "rough ride" , "Wild horse of W. US" , "An unbroken mustang, bucking perhaps" , "Untamed mustang" , "Wild horse of the West" .)