Recommended sextet to board commercial jet (9)
I believe the answer is:
advisable
'recommended' is the definition.
(similar in meaning)
'sextet to board commercial jet' is the wordplay.
'sextet' becomes 'vi' (group of six - VI are the Roman numerals for six).
'to board' is an insertion indicator (inserted letters come on board).
'commercial' becomes 'ad' (abbreviation for advertisement).
'jet' becomes 'sable' (both can mean a black colour).
'ad'+'sable'='adsable'
'vi' placed inside 'adsable' is 'ADVISABLE'.
(Other definitions for advisable that I've seen before include "Prudent - recommended" , "Wise, recommended" .)