Tallest soldier set off after support? (8)
I believe the answer is:
leggiest
'tallest' is the definition.
('leggy' can be similar in meaning to 'tall')
'soldier set off after support?' is the wordplay.
'soldier' becomes 'GI' (term for US soldier).
'off' is an anagram indicator.
'after' says to put letters next to each other.
'support?' becomes 'leg' (leg is a kind of support).
'set' with letters rearranged gives 'est'.
'gi'+'est'='giest'
'giest' after 'leg' is 'LEGGIEST'.
(Another definition for leggiest that I've seen is " Having the longest lower limbs".)