Young Girl Guide initially welcomed by supporter on course (7)
I believe the answer is:
teenage
'young' is the definition.
(similar in meaning)
'girl guide initially welcomed by supporter on course' is the wordplay.
'girl' becomes 'ena' (woman's name).
'guide initially' becomes 'g' (I can't explain this - if you can you should believe this answer much more).
'welcomed by' means one lot of letters goes inside another (letters welcomed inside).
'supporter on course' becomes 'tee' (a tee supports a ball on a golf course).
'ena'+'g'='enag'
'enag' inserted within 'tee' is 'TEENAGE'.
Can you help me to learn more?
(Other definitions for teenage that I've seen before include "Juvenile" , "Like a category of youngsters" , "Eat gene to become young" , "13 to 19 years old" , "Adolescent" .)