Start badly but get a jewel; that's the plan (9)
I believe the answer is:
stratagem
'the plan' is the definition.
(stratagem is a kind of plan)
'start badly but get a jewel' is the wordplay.
'badly' indicates anagramming the letters (letters in the wrong order).
'but' becomes 'a' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'get' indicates putting letters inside.
'a jewel' becomes 'gem' ('gem' can be a synonym of 'jewel').
'start' is an anagram of 'strta'.
'strta' enclosing 'a' is 'strata'.
'strata'+'gem'='STRATAGEM'
'that's' acts as a link.
Can you help me to learn more?
(Other definitions for stratagem that I've seen before include "Cunning manoeuvre" , "A cunning plan or scheme" , "Cunning plot" , "Plan to deceive enemy" , "Ploy - start game (anag)" .)