Old head of English pinching cash machine -- a student gets porridge! (7)
I believe the answer is:
oatmeal
'porridge' is the definition.
('oatmeal' can be a synonym of 'porridge')
'old head of english pinching cash machine a student' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'head of' says to take the initial letters.
'pinching' means one lot of letters goes inside another (taking inside).
'cash machine' becomes 'atm' ('ATM' can be a synonym of 'cash machine').
'student' becomes 'l' (as in L-plates for learner drivers).
The initial letter of 'english' is 'e'.
'o'+'e'='oe'
'oe' enclosing 'atm' is 'oatme'.
'oatme'+'a'+'l'='OATMEAL'
'gets' acts as a link.
(Other definitions for oatmeal that I've seen before include "Ground grain used to make porridge" , "Kind of porridge" , "The material for porridge" , "Plain food" , "Porridge ingredient" .)