Make modifications to ’arness (5)
I believe the answer is:
alter
This clue is a double definition.
'make modifications' is the first definition.
(I've seen this in another clue)
''arness' is the second definition.
('halter' - thesaurus - said with the 'H' dropped, a cockney accent, like 'harness' was)
'to' is the link.
(Other definitions for alter that I've seen before include "Adapt" , "Vary" , "Metamorphose" , "convert" , "reform" .)