Green endpiece in Rex’s jumble (13)
I believe the answer is:
inexperienced
'green' is the definition.
(similar in meaning)
'endpiece in rex's jumble' is the wordplay.
'jumble' indicates anagramming the letters.
'endpiece'+'in'+'rex'='endpieceinrex'
'endpieceinrex' is an anagram of 'INEXPERIENCED'.
(Other definitions for inexperienced that I've seen before include "Raw, unskilled" , "Raw, green" , "Green, naive" , "Callow" , "unqualified" .)