The struggle to the first on Earth, possibly? (6)
I believe the answer is:
strife
'the struggle' is the definition.
(strife is a kind of struggle)
'first on earth possibly?' is the wordplay.
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'earth' becomes 'E'.
'possibly?' indicates anagramming the letters (I've seen 'possibly' mean this).
'first'+'e'='firste'
'firste' is an anagram of 'STRIFE'.
'to the' is the link.
(Other definitions for strife that I've seen before include "'Discord, conflict (6)'" , "Trouble and discord" , "Lack of harmony" , "Heated discord" , "Discord and friction" .)