Small illustration, one fed into computer (3)
I believe the answer is:
pic
'small illustration' is the definition.
(short for picture; 'small' suggests an abbreviation)
'one fed into computer' is the wordplay.
'one' becomes 'i' (Roman numeral).
'fed into' is an insertion indicator.
'computer' becomes 'PC' (personal computer).
'i' placed within 'pc' is 'PIC'.
(Other definitions for pic that I've seen before include "Photograph (colloq.)" , "Image (abbr)" , "Photo (informal)" , "illustration" , "Photo (colloq.)" .)