Understand how to acquire sex appeal (3,2)
I believe the answer is:
get it
'understand' is the definition.
(I've seen this before)
'to acquire sex appeal' is the wordplay.
'to acquire' becomes 'get' ('get' can be a synonym of 'acquire').
'sex appeal' becomes 'it'.
'get'+'it'='GET IT?'
'how' is the link.
(Other definitions for get it that I've seen before include "twig?" , "Understand" , "See the point" , "Be punished" .)