Absent-minded son races through coral reef (6)
I believe the answer is:
scatty
'absent-minded' is the definition.
(I've seen this before)
'son races through coral reef' is the wordplay.
'son' becomes 's'.
'races' becomes 'tt' (Isle of Man Tourist Trophy races).
'through' means one lot of letters goes inside another (I've seen this in other clues).
'coral reef' becomes 'cay' (cay is a kind of coral reef).
'tt' inserted inside 'cay' is 'catty'.
's'+'catty'='SCATTY'
(Other definitions for scatty that I've seen before include "Informally silly or crazy" , "Hare-brained" , "Distracted" , "Disorganised, unpredictable" , "Not thinking straight" .)