Thus lawful alternative for legal advisor (9)
I believe the answer is:
solicitor
'legal advisor' is the definition.
(I've seen this before)
'thus lawful alternative' is the wordplay.
'thus' becomes 'so' (synonyms).
'lawful' becomes 'licit' (synonyms).
'alternative' becomes 'or'.
'so'+'licit'+'or'='SOLICITOR'
'for' is the link.
(Other definitions for solicitor that I've seen before include "Court player" , "Legal officer" , "Type of lawyer" , "Legal advisor" , "legal expert" .)