Hanger-on finding resistance in balance disturbed (8)
I believe the answer is:
barnacle
'hanger-on' is the definition.
(thesaurus)
'resistance in balance disturbed' is the wordplay.
'resistance' becomes 'r' (abbreviation).
'in' means one lot of letters goes inside another.
'disturbed' indicates anagramming the letters.
'balance' is an anagram of 'banacle'.
'r' put into 'banacle' is 'BARNACLE'.
'finding' acts as a link.
(Other definitions for barnacle that I've seen before include "Shellfish that sticks to things" , "A shellfish that sticks to hulls etc" , "Crustacean that adheres to ships' bottom" , "Crustacean that attaches to submerged surfaces" , "'Marine crustacean, attaches to a boat-hull (8)'" .)