Understands to be a bread sauce ingredient (5)
I believe the answer is:
reads
'understands' is the definition.
(reading is a kind of understanding)
'a bread sauce ingredient' is the wordplay.
'ingredient' says the answer is hidden in the clue (I've seen 'ingredient of' mean this).
'READS' can be found hidden inside 'a bread sauce'.
'to be' is the link.
(Other definitions for reads that I've seen before include "Studies (at university)" , "Peruses" , "Looks at and undertsands the written word" , "Understands written language" , "Scans" .)