Consider it a self-inflicted disadvantage to possess purpose, perhaps? (3,4)
I believe the answer is:
own goal
'consider it a self-inflicted disadvantage' is the definition.
I can't judge whether this defines the answer.
'possess purpose perhaps?' is the wordplay.
'possess' becomes 'own' (synonyms).
'purpose perhaps?' becomes 'goal' (purpose is a kind of goal).
'own'+'goal'='OWN GOAL'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for own goal that I've seen before include "Have a target" , "Blunder penalising the perpetrator" , "Action that backfires" , "embarrassing error" , "Unintentional self-inflicted harm" .)