Cereal company supported by sailors (4)
I believe the answer is:
corn
'cereal' is the definition.
(corn is a kind of cereal)
'company supported by sailors' is the wordplay.
'company' becomes 'co' (abbreviation).
'supported by' says to put letters next to each other (letters go below - in down clue).
'sailors' becomes 'rn' (abbreviation for Royal Navy).
'co'+'rn'='CORN'
(Other definitions for corn that I've seen before include "Something unoriginal - cereal plant" , "Painful affliction" , "Grain on a toe?" , "Maize" , "Grain; hard skin" .)