Hard — of hearing? (6)
I believe the answer is:
Here is my best explanation:
This clue is a double definition.
'hard' is the first definition.
('trying' can be similar in meaning to 'hard')
'hearing' is the second definition.
('try' can be a synonym of 'hear')
'of' is the link.
(Other definitions for trying that I've seen before include "occupation of Oliver Wendell Holmes Jnr", "Sitting in judgement", "Aggravating", "What could be witnessed in court", "work on bench", "distressing", "Vexatious - having a go", "Straining one's patience", "Testing" and "Irksome".)