Whip is extremely keen - when available (5)
I believe the answer is:
knout
'whip' is the definition.
(I know that knout is a type of whip)
'extremely keen when available' is the wordplay.
'extremely' says to hollow out the word (remove centre letters) (only the extremes of the word are used).
'when available' becomes 'out' (as in a product which is out. I am not sure about the 'when' bit.).
'keen' with its middle removed is 'kn'.
'kn'+'out'='KNOUT'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for knout that I've seen before include "Whip once used in Russia" , "Russian whip is finally not at home" , "It was used to inflict punishment" , "Flog" .)