Heading for the main prize after serious effort initially (7)
I believe the answer is:
seaward
'heading for the main' is the definition.
('the main' is an old name for the sea)
'prize after serious effort initially' is the wordplay.
'prize' becomes 'award' (synonyms).
'after' says to put letters next to each other.
'initially' suggests taking the first letters.
The initial letters of 'serious effort' is 'se'.
'award' put after 'se' is 'SEAWARD'.
(Other definitions for seaward that I've seen before include "Like sailor going out" , "Towards the ocean" , "heading for coast?" .)