Out of practice, head of firm's removed (5)
I believe the answer is:
rusty
'out of practice' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for rusty that I've seen before include "Oxidised, corroded" , "a bit faded" , "Behind the times" , "Stiff with age" , "(Of iron) affected by damp" .)