Hard — of hearing? (6)
I believe the answer is:
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' acts as a link.
(Other definitions for trying that I've seen before include "Taxing" , "Annoying; sampling" , "Vexatious - having a go" , "Hearing (a case)" , "occupation of Oliver Wendell Holmes Jnr" .)