Ace group taking leave (7)
I believe the answer is:
abandon
'leave' is the definition.
(to abandon is to leave or desert)
'ace group taking' is the wordplay.
'ace' becomes 'a' (abbreviation).
'group' becomes 'band' (band can mean a group of people).
'taking' becomes 'on' (eg one taking drugs is 'on' drugs).
'a'+'band'+'on'='ABANDON'
(Other definitions for abandon that I've seen before include "Wantonness: desert" , "Total lack of inhibition (often gay?)" , "Stop" , "Leave high and dry" , "Desert; lack of restraint" .)