Skill I learnt initially in prison which some find hard to swallow (9)
I believe the answer is:
cartilage
'which some find hard to swallow' is the definition.
(tough or hard to swallow gristle in meat)
'skill i learnt initially in prison' is the wordplay.
'skill' becomes 'art' (both can mean a talent).
'initially' indicates taking the first letters.
'in' indicates putting letters inside.
'prison' becomes 'cage' (I've seen this before).
The initial letter of 'learnt' is 'l'.
'art'+'i'+'l'='artil'
'artil' inserted within 'cage' is 'CARTILAGE'.
(Other definitions for cartilage that I've seen before include "Connective tissue in joints" , "bodily tissue" , "Gristle" , "Tough white flexible tissue attached to bones" .)