Oliver Twist, say — he's first seen in ragged apron (6)
I believe the answer is:
orphan
'oliver twist say he's first seen in ragged apron' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'he's first seen in ragged apron' is the wordplay.
'he's first' becomes 'h' (1st letter of 'he').
'seen in' means one lot of letters goes inside another.
'ragged' indicates anagramming the letters.
'apron' is an anagram of 'orpan'.
'h' going within 'orpan' is 'ORPHAN'.
Can you help me to learn more?
(Other definitions for orphan that I've seen before include "Child who has lost both parents" , "One deprived" , "Child without parent" , "Bereaved child" , "One bereft of parents" .)