Leave a group aboard (7)
I believe the answer is:
abandon
'leave' is the definition.
(to abandon is to leave or desert)
'a group aboard' is the wordplay.
'group' becomes 'band' (band can mean a group of people).
'aboard' becomes 'on'.
'a'+'band'+'on'='ABANDON'
(Other definitions for abandon that I've seen before include "Maroon" , "Desert, strand" , "a complete lack of restraint" , "Desert; intemperance" , "scrap" .)