Fully understand some cereal is expensive (7)
I believe the answer is:
realise
'fully understand' is the definition.
'realise' can be an answer for 'understand' (I've seen this before). I'm not sure about the 'fully' bit.
'some cereal is expensive' is the wordplay.
'some' indicates the answer is hidden within the clue (some parts of the word are used).
'REALISE' is hidden in the letters of 'cereal is expensive'.
Can you help me to learn more?
(Other definitions for realise that I've seen before include "Convert (asset) into cash" , "effect" , "Convert assets into cash" , "net" , "Convert goods or property into cash" .)