Reliable, but ultimately out of practice (6)
I believe the answer is:
trusty
'reliable' is the definition.
('trusty' is associated in meaning with 'reliable')
'but ultimately out of practice' is the wordplay.
'ultimately' indicates one should take the final letters.
'out of practice' becomes 'rusty' ('rusty' can be a synonym of 'out of practice').
The last letter of 'but' is 't'.
't'+'rusty'='TRUSTY'
(Other definitions for trusty that I've seen before include "Devoted" , "Reliable (prisoner)" , "Staunch" , "Trainee officer" , "Responsible prisoner" .)