Having long standing supporters (5)
I believe the answer is:
'long standing supporters' is the definition.
'leggy' can be an answer for 'long' (thesaurus). I am unsure of the 'standing supporters' bit.
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for leggy that I've seen before include "(Of a plant) tall and straggly" , "Attractively tall?" , "(Of plant) having a long stem" , "Long, slender" , "with long pins" .)