A supporter legally without a friend (3)
I believe the answer is:
Here is my best explanation:
'a supporter' is the definition.
'legally without a friend' is the subsidiary indication.
'without' suggests deleting specific letters.
'a friend' becomes 'ally' ('ally' can be a synonym of 'friend').
'legally' with 'ally' removed is 'leg'.
(Other definitions for leg that I've seen before include "Poultry portion", "Section of journey; limb", "part of course", "Stage (of race)", "prop", "On", "Support; cricket side", "Projecting support for chair or table", "Limb; stage" and "A lower limb".)