Out of memory on mainframe's core (4)
I believe the answer is:
from
'out of' is the definition.
(I've seen this in another clue)
'memory on mainframe's core' is the wordplay.
'memory' becomes 'rom' (ROM is an example).
'on' means one lot of letters go next to another.
'core' says to take the centre.
The middle letter of 'mainframe' is 'f'.
'rom' put after 'f' is 'FROM'.
(Other definitions for from that I've seen before include "Springing out of" , "Beginning at; out of" , "Sent by" .)