Wrote of poetry and briefly about legal document (5,2,8)
I believe the answer is:
power of attorney
'legal document' is the definition.
(power of attorney is a kind of legal document)
'wrote of poetry and briefly about' is the wordplay.
'briefly' means to remove the last letter.
'about' indicates anagramming the letters.
'wrote'+'of'+'poetry'+'and'='wroteofpoetryand'
'wroteofpoetryand' with its final letter taken away is 'wroteofpoetryan'.
'wroteofpoetryan' is an anagram of 'POWER OF ATTORNEY'.
(Other definitions for power of attorney that I've seen before include "legal authority" , "Agent's authority" , "sufficient for deed" , "American lawyer's energy" .)