Part in this way, missing ex at first (4)
I believe the answer is:
some
'part' is the definition.
(thesaurus)
'this way missing ex at first' is the wordplay.
'this way' becomes 'so' (I've seen this in other clues**).
'at first' says to take the initial letters.
The initial letters of 'missing ex' is 'me'.
'so'+'me'='SOME'
'in' acts as a link.
(Other definitions for some that I've seen before include "An undetermined number" , "Undefined number" , "An unspecified quantity, not all" , "A little; not a little" , "certain" .)