Jack left after pocketing fantastic wee gem (5)
I believe the answer is:
jewel
'gem' is the definition.
(synonyms)
'jack left after pocketing fantastic wee' is the wordplay.
'jack' becomes 'J' (playing card abbreviation).
'left' becomes 'l' (common abbreviation).
'after' says to put letters next to each other.
'pocketing' is an insertion indicator (pocket can mean to steal or take possession of).
'fantastic' is an anagram indicator (a fantastic or improbable ordering of the letters).
'j'+'l'='jl'
'wee' is an anagram of 'ewe'.
'jl' going around 'ewe' is 'JEWEL'.
(Other definitions for jewel that I've seen before include "Gem" , "Gemstone" , "eg Ruby" , "Precious stone or mineral" , "The - That Was Ours (Dexter)" .)