The main feature of the first dozen letters on line (5)
I believe the answer is:
atoll
'the main feature' is the definition.
(reef in the sea - 'the main' is a name for the sea)
'first dozen letters on line' is the wordplay.
'first dozen letters' becomes 'A to L' (first 12 letters of the alphabet).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'line' becomes 'l' (used when specifying particular lines from a poem).
'atol'+'l'='ATOLL'
'of the' acts as a link.
(Other definitions for atoll that I've seen before include "Circular coral reef" , "Bikini for one" , "Ring of coral" , "Coral island(s) surrounding a lagoon" , "tropical paradise?" .)