Separate small youngster gets illuminated (5,2)
I believe the answer is:
split up
'separate' is the definition.
(I know that separate can be written as split up)
'small youngster gets illuminated' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'youngster' becomes 'pup' (a pup is a young dog).
'gets' is an insertion indicator (gets can mean captures or absorbs).
'illuminated' becomes 'lit' (synonyms).
'pup' enclosing 'lit' is 'plitup'.
's'+'plitup'='SPLIT UP'
(Other definitions for split up that I've seen before include "End a relationship" , "Separate" , "Take apart" , "Divorce" , "Part company" .)