Some East-ender is eating mash? (9)
I believe the answer is:
tenderise
'mash?' is the definition.
('mash'->'tenderize' is in my internal thesaurus and 'ize'->'ise')
'some east-ender is eating' is the wordplay.
'some' indicates the answer is hidden within the clue (some of the parts of the word are used).
'TENDERISE' can be found hidden inside 'east ender is eating'.
(Other definitions for tenderise that I've seen before include "pound" , "soften flesh" .)