Plaintiff makes appeal to HM (10)
I believe the answer is:
petitioner
'plaintiff' is the definition.
(thesaurus)
'appeal to hm' is the wordplay.
'appeal' becomes 'petition' (I've seen this before).
'to hm' becomes 'er' (HM Her Majesty, ER Elizabeth Regina).
'petition'+'er'='PETITIONER'
'makes' acts as a link.
(Other definitions for petitioner that I've seen before include "One with a suit" , "One making appeal to authority" .)