Hi Fop,
15d Your letters are correct but the 3rd is O
20d Initial/a little U(nwisely) + 7 letter word meaning "skull cap" minus it's first letter - definition is 'heavy metal'.
21a Definition is "Alert" and it's a homophone clue - first is a homophone of what you call when you score a boundary in cricket and the second word is a homophone of a previous captain of Australia (cricket)