Pretentious girl going after young man (2-2-2)
I believe the answer is:
la-di-da
'pretentious' is the definition.
(I've seen this before)
'girl going after young man' is the wordplay.
'girl' becomes 'Ida'.
'going after' means one lot of letters go next to another.
'young man' becomes 'lad' (lad can mean a boy or young man).
'ida' after 'lad' is 'LA-DI-DA'.
(Other definitions for la-di-da that I've seen before include "Pretentious, snobbish" , "I'd a lad with affected manner or pronunciation" , "Hoity-toity" , "Stuck-up" , "Informally, having affected manner of pronunciation" .)