The bosun's porridge ingredient (4)

I believe the answer is:
oats
'porridge ingredient' is the definition.
(I've seen this in another clue)
'the bosun's' is the wordplay.
I cannot really understand how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is located in the answer.
The remaining letters 'oas' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for oats that I've seen before include "Cereal grass and seeds" , "Sow wild ones in disreputable youth" , "Seed grasses" , "Cereal plants" , "Porridge ingredient" .)
