Obtain what’s needed, do you understand? (3,2)
I believe the answer is:
get it
'do you understand?' is the definition.
(I've seen this before)
'obtain what's needed' is the wordplay.
'obtain' becomes 'get' (synonyms).
'what's needed' becomes 'it' (that's it!).
'get'+'it'='GET IT?'
(Other definitions for get it that I've seen before include "Understand" , "twig?" , "See the point" , "Be punished" .)