Following summer in Marseille, sailors will have supporter for all time (9)
I believe the answer is:
eternally
'for all time' is the definition.
(I've seen this in another clue)
'following summer in marseille sailors will have supporter' is the wordplay.
'following' says to put letters next to each other.
'summer in marseille' becomes 'ete' (I've seen this in another clue).
'sailors' becomes 'RN' (Royal Navy).
'will have' is a charade indicator (letters next to each other).
'supporter' becomes 'ally' (an ally is a friend or supporter).
'ete' next to 'rn' is 'etern'.
'etern'+'ally'='ETERNALLY'
(Other definitions for eternally that I've seen before include "For evermore" , "all the time" , "Always and forever" , "constantly" , "Nearly let out forever" .)