Brief clue son cracked (7)
I believe the answer is:
counsel
'brief' is the definition.
(thesaurus)
'clue son cracked' is the wordplay.
'cracked' indicates anagramming the letters.
'clue'+'son'='clueson'
'clueson' with letters rearranged gives 'COUNSEL'.
(Other definitions for counsel that I've seen before include "Advocate" , "Lawyer who pleads cases in court" , "kept by the uncommunicative" , "In court brief" , "Guidance" .)