Reliable pianist ultimately out of practice (6)
I believe the answer is:
trusty
'reliable' is the definition.
(similar in meaning)
'pianist ultimately out of practice' is the wordplay.
'ultimately' suggests the final letters.
'out of practice' becomes 'rusty' (synonyms).
The last letter of 'pianist' is 't'.
't'+'rusty'='TRUSTY'
(Other definitions for trusty that I've seen before include "Devoted" , "Reliable, honest and firm" , "Responsible prisoner" , "Reliable (prisoner)" , "Dependable" .)