Incompletely operate part of farm being out of practice (5)
I believe the answer is:
rusty
'out of practice' is the definition.
(synonyms)
'incompletely operate part of farm' is the wordplay.
'incompletely' means to remove the last letter.
'operate' becomes 'run' (running is a kind of operating).
'part of farm' becomes 'sty' (I am not sure about this - if you are sure you should believe this answer much more).
'run' with its final letter taken away is 'ru'.
'ru'+'sty'='RUSTY'
'being' acts as a link.
Can you help me to learn more?
(Other definitions for rusty that I've seen before include "(Of iron) corroded" , "Behind the times" , "a bit faded" , "(Of knowledge) not current" , "Oxidised, corroded" .)