Lacking polish, being out of practice (5)
I believe the answer is:
rusty
'out of practice' is the definition.
(synonyms)
'lacking polish being' is the wordplay.
'lacking' means to remove the last letter (the word is incomplete or lacking).
'polish' becomes 'rub' (polish meaning rub to make shiny).
'being' becomes 'sty' (I can't explain this - if you can you should give a lot more credence to this answer).
'rub' with its final letter taken away is 'ru'.
'ru'+'sty'='RUSTY'
Can you help me to learn more?
(Other definitions for rusty that I've seen before include "Oxidised, corroded" , "Having a reddish-orange oxide coating" , "affected by inactivity" , "(Of knowledge) not current" , "Affected by corrosion - out of practice" .)