Forget it? Number always on memory (5,4)
I believe the answer is:
never mind
'forget it?' is the definition.
(thesaurus)
'number always on memory' is the wordplay.
'number' becomes 'n'.
'always' becomes 'ever' ('ever' can be a synonym of 'always').
'on' is a charade indicator (letters next to each other).
'memory' becomes 'mind' (mind is a kind of memory).
'n'+'ever'+'mind'='NEVER MIND'
(Other definitions for never mind that I've seen before include "Forget" , "who cares?" , "Don't worry" , "It doesn't matter" .)