Sponging soldiers getting one to twitch (9)
I believe the answer is:
parasitic
'sponging' is the definition.
(I've seen this before)
'soldiers getting one to twitch' is the wordplay.
'soldiers' becomes 'paras' (para is a kind of soldier).
'getting' is a charade indicator (letters next to each other).
'one' becomes 'I' (Roman numeral).
'to twitch' becomes 'tic' (tic is a kind of twitch).
'paras'+'i'+'tic'='PARASITIC'
(Other definitions for parasitic that I've seen before include "Like mistletoe" , "like a leech" , "to be paid for by someone else" , "Living at the expense of the host organism" , "Sponging" .)