Neglect one on purpose (7)
I believe the answer is:
mission
'purpose' is the definition.
(thesaurus)
'neglect one on' is the wordplay.
'neglect' becomes 'miss' (missing is a kind of neglecting).
'one' becomes 'I' (Roman numeral).
'miss'+'i'+'on'='MISSION'
(Other definitions for mission that I've seen before include "Ministry sent by a church to spread its teaching" , "In religious campaign" , "Important assignment" , "Vocation; embassy" , "Delegation" .)