Admit girl gets old - that's self-inflicted (3,4)
I believe the answer is:
own goal
'self-inflicted' is the definition.
Both the definition and answer are adjectives.
Perhaps there's a link between them I don't understand?
'admit girl gets old' is the wordplay.
'admit' becomes 'own' (own can mean to admit or acknowledge).
'girl' becomes 'gal' ().
'gets' is an insertion indicator (gets can mean captures or absorbs).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'gal' going around 'o' is 'goal'.
'own'+'goal'='OWN GOAL'
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for own goal that I've seen before include "Self-inflicted damage" , "could be self-defeating" , "Blunder penalising the perpetrator" , "bad mistake" , "(Soccer) score which helps opposing team" .)