Son growing to three times the size of youth (9)
I believe the answer is:
stripling
'youth' is the definition.
(I've seen this before)
'son growing to three' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'growing' becomes 'pling' (I can't explain this - if you can you should believe this answer much more).
'to' says to put letters next to each other (I've seen this in other clues).
'three' becomes 'tri' (prefix meaning 'three').
'pling' put after 'tri' is 'tripling'.
's'+'tripling'='STRIPLING'
'times the size of' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for stripling that I've seen before include "Young boy" , "Young man" , "Youth" , "Lad" , "youngster" .)